Abstract: 10Ni5CrMoV steel is widely used in ship structure.In order to eliminate the remain stress yielded in weld process,the deposit metal usually is heat-treated after welding.It is found that the-50℃ impact energy of the welding deposit metal after-welding heat treatment descends remarkably.SEM、TEM and AES were employed to investigate the microstructure and impact fracture energy.The results show that disappearance of batten structure during the treatment,segregation of phosphorus on grain boundary and large amount of Fe3C precipitated on grain boundary lead the descent of impact energy.