Cheats:

Cheat mode:
Hold L + B + R (in that order) at the main menu, then release B, R, L (in that order) to display the code entry screen.

Play as Destroyah:
Enter 537084 as a code. A monster will roar to confirm correct code entry. Alternately, successfully complete adventure mode as Godzilla 2000.

Play as Gigan:
Enter 616233 as a code. A monster will roar to confirm correct code entry. Alternately, successfully complete adventure mode as Aguirus.

Play as Godzilla 2000:
Enter 225133 as a code. A monster will roar to confirm correct code entry. Alternately, successfully complete adventure mode as Godzilla 90s version.

Play as King Ghidrah:
Enter 877467 as a code. A monster will roar to confirm correct code entry. Alternately, successfully complete adventure mode as Megalon.

Play as Mecha Godzilla:
Enter 131008 as a code. A monster will roar to confirm correct code entry. Alternately, successfully complete adventure mode as Destoroyah, Rodan and Mecha King Ghidrah

Play as Mecha King Ghidrah:
Enter 557456 as a code. A monster will roar to confirm correct code entry. Alternately, successfully complete adventure mode with King Ghidrah.

Play as Orga:
Enter 202412 as a code. A monster will roar to confirm correct code entry. Alternately, successfully complete adventure mode with all characters once, then complete adventure mode again under the medium difficulty setting as Godzilla 2000.

Play as Rodan:
Enter 104332 as a code. A monster will roar to confirm correct code entry. Alternately, successfully complete adventure mode as Gigan.

Almost all monsters:
Enter 696924 as a code. A monster will roar to confirm correct code entry. All monsters except for Orga will be unlocked.

Technicolor mode:
Enter 661334 as a code. A monster will roar to confirm correct code entry.

Black and white mode:
Enter 567980 as a code. A monster will roar to confirm correct code entry.

Smog mode:
Enter 913963 as a code. A monster will roar to confirm correct code entry.

11 continues in adventure mode:
Enter 760611 as a code. A monster will roar to confirm correct code entry.

All cities:
Enter 480148 as a code. A monster will roar to confirm correct code entry.

Indestructible buildings:
Enter 112122 as a code. A monster will roar to confirm correct code entry.

All buildings and objects throwable:
Enter 756287 as a code. A monster will roar to confirm correct code entry.

Small player one:
Enter 986875 as a code. A monster will roar to confirm correct code entry.

Small player two:
Enter 971934 as a code. A monster will roar to confirm correct code entry.

Small player three:
Enter 895636 as a code. A monster will roar to confirm correct code entry.

Small player four:
Enter 795735 as a code. A monster will roar to confirm correct code entry.

Small players:
Enter 174204 as a code. A monster will roar to confirm correct code entry.

Invisible player two:
Enter 459113 as a code. A monster will roar to confirm correct code entry.

Invisible players:
Enter 316022 as a code. A monster will roar to confirm correct code entry.

Unlimited rage for player one:
Enter 649640 as a code. A monster will roar to confirm correct code entry.

Unlimited rage for player two:
Enter 122224 as a code. A monster will roar to confirm correct code entry.

Unlimited rage for player three:
Enter 548053 as a code. A monster will roar to confirm correct code entry.

Unlimited rage for player four:
Enter 451242 as a code. A monster will roar to confirm correct code entry.

No damage for player one:
Enter 843901 as a code. A monster will roar to confirm correct code entry.

No damage for player two:
Enter 706149 as a code. A monster will roar to confirm correct code entry.

No damage for player three:
Enter 188522 as a code. A monster will roar to confirm correct code entry.

No damage for player four:
Enter 286552 as a code. A monster will roar to confirm correct code entry.

No damage for all players:
Enter 505634 as a code. A monster will roar to confirm correct code entry.

Extra damage done by player one:
Enter 511012 as a code. A monster will roar to confirm correct code entry.

Extra damage done by player two:
Enter 815480 as a code. A monster will roar to confirm correct code entry.

Extra damage done by player three:
Enter 212454 as a code. A monster will roar to confirm correct code entry.

Extra damage done by player four:
Enter 286552 as a code. A monster will roar to confirm correct code entry.

Extra damage done by all players:
Enter 817683 as a code. A monster will roar to confirm correct code entry.

Regenerate energy:
Enter 492877 as a code. A monster will roar to confirm correct code entry.

Energy boost for player one:
Enter 650867 as a code. A monster will roar to confirm correct code entry.

Unlimited energy for player one:
Enter 677251 as a code. A monster will roar to confirm correct code entry.

Unlimited energy for player two:
Enter 435976 as a code. A monster will roar to confirm correct code entry.

Unlimited energy for player three:
Enter 603696 as a code. A monster will roar to confirm correct code entry.

Unlimited energy for player four:
Enter 291680 as a code. A monster will roar to confirm correct code entry.

Toggle military response:
Enter 256806 as a code. A monster will roar to confirm correct code entry.

Extra damage done by military:
Enter 308929 as a code. A monster will roar to confirm correct code entry.

Player status always displayed:
Enter 135984 as a code. A monster will roar to confirm correct code entry.

Disable life and energy display:
Enter 443253 as a code. A monster will roar to confirm correct code entry.

Disable health power-ups:
Enter 562142 as a code. A monster will roar to confirm correct code entry.

Disable energy power-ups:
Enter 413403 as a code. A monster will roar to confirm correct code entry.

Disable rage power-ups:
Enter 119702 as a code. A monster will roar to confirm correct code entry.

Disable Mothra power-ups:
Enter 134615 as a code. A monster will roar to confirm correct code entry.

Disable freeze tanks:
Enter 841720 as a code. A monster will roar to confirm correct code entry.

Telephone:
Enter 465472 as a code. A monster will roar to confirm correct code entry.

View credits:
Enter 176542 as a code. A monster will roar to confirm correct code entry.

View version number:
Enter 097401 as a code. A monster will roar to confirm correct code entry.

Mothership level:
Successfully complete adventure mode as Mecha Godzilla.

Hint: Unlocking monsters:
Successfully complete adventure mode with a monster to unlock other monsters. To unlock the last monster, complete adventure mode with Godzilla 2000.

Hint: Taunt:
To taunt, press B + X or A + X.

Hint: Special moves:
Moves that are special to some monsters can be executed by pressing R + L or A + Y. For example, Gigan teleports and Megalon digs underground. Note: While underground with Megalon you have to continue holding these buttons to move around. Target yourself under your opponent and execute a grab. You will emerge from the ground with the opponent in your hands. If you miss, you will be standing open to attack because you are exhausted from digging.

Hint: Picture Gallery:
To unlock pictures in the Gallery, play through adventure mode under the medium or hard difficulty setting. Some buildings that you destroy will contain Silver Atari symbols. Collect them to unlock a picture.

For Walkthroughs Try These Sites:

www.gamespot.com

www.cheatplanet.com