The corrosion resistances of 01420 Al-Li alloy under aged and pesk aged conditions have been studied systematically. The results show that the corrosion resistances of under aged alloy are better than those of peak aged alloy for intergranular corrosion, exfoliationcorrosion, salt spray corrosion and stress corrosion testing. The changes of corrosion resistance of alloy owe to the variations of quantity and distribution of S phase. The corrosion resistance of alloy will gradually lower with the increasment of S phase and its distribution in grain boundary.