7.62x39mm originally. There are newer AK variants that use 5.45x39mm.
Kalashnikov was against the 5.4mm round, stating that it had less stopping power as the original 7.62mm. He was over ruled by the Soviet designers who thought why should the US adopt a 5.56mm round if it is too small. It turns out in Iraq the 5.56mm is too small. The soldier may be able to carry more 5.56mm versus the larger 7.62mm round, but if it takes three direct hits to drop the target versus one hit from a 7.62mm, the advantage of carrying more smaller rounds become moot.
The AK and its variants have been produced in a number of calibers:
.22 trainers
7.62x39 the original AK 47
5.45x39 the AK 74
5.45x45 which uses our .223 round
7.62x51 also known as nato.308
7.62x54
8mm mauser used in the yugo m76
410 shotgun
20gau shotgun
and
12gau shotgun
Yep. M. Kalashnikov never approved of the switch in calibers. He was probably correct.