This　work　investigated　the　oxidation　behaviour　of　18-8　austenitic　stainless　steel　under　cyclic　steam　oxidation　condition.　The　results　indicate　that　grain　boundaries　not　only　promote　the　chromium　outward　diffusion,　but　also　provide　the　fast　diffusion　paths　for　the　oxygen　penetration.　Thus,　oxide　inward　growth　occurs　when　the　chromium　content　at　scale-matrix　interface　cannot　sustain　the　continuous　chromia　scale.　In　addition,　the　grain　boundaries　promote　the　Fe　outward　diffusion,　accompanied　with　the　fast　growth　of　interfacial　voids　between　two　oxide　layers.　As　a　result,　the　oxide　scale　exfoliates　once　the　voids　grow　to　a　critical　value.