Crack initiation testing were performed both in high temperature water and gas environments using blunt notch compact tension type specimens in the range of temperature between 450 to 320℃ using CW TT690 and CW Carbon steel.Intergranular cracking initiated from the bottom of notch both materials.Cavity was observed at high stress site(just below the notch)before crack initiation in both materials.Judging from the measured distribution of cavity,the cavity seems to be formed result from the diffusion of cold worked induced a-thermal vacancy driven by stress gradient.And then,results of both materials were compared with results of CW MA600 and CW 800,CW316 to consider the material dependence on kinetics of cavity formation.Most rapid cavity formation was observed in bcc material(CW carbon steel).Among in same fcc materials,much more rapid cavity formation was observed in CW TT690 and CW 600 than CW 800 and CW316.The results suggested that rapid diffusion did occur in the materials even at LWR operating temperature if material was enough cold worked during long terms operation.Based on the results,possible changes in grain boundary compositions in CW TT690 were examined and will be discussed the effect on SCC initiation after long terms operation.