Abstract: The microstructure of 10Ni5CrMoV steel was observed by optical microscopy and the effects of heating temperature, heating time and circular times on grain size were discussed after the steel was heated for different times at 930-1030 ℃ and quenched for different circular times, and mechanical properties and fracture morphology before and after heat treatment of the steel were analyzed. The results show that grain size of the steel increased with prolonging of the heating time at different temperatures, the grain size was best when heating time was 100 s. The grain was coarse and heterogeneous when circular times were less than three times, and the refining effect was not obvious when circular times were more than three times. Raising the heating temperature, thegrain size decreased and then increased, and it was finest at 950 ℃. The optimal process of grain refining was heating temperature of 950 ℃, heating time of 100 s and circular quenching times of three times, and the grain could refine from 253 μm to 63 μm. The strength and ductility could be raised obviously after quenching and temgering.