Abstract: The critical temperatures of 30CrNiMo8 steel were determined by dilatometer method combined with metallographic test and hardness test on a thermal dilatometer. Also, the continuous cooling transformation curves of super-cooled austenite were obtained. The influence of different cooling rates on microstructure and hardness of 30CrNiMo8 steel was studied. The results show that:with the increase of cooling rate between 0.02-7℃ ·s^-1, the microstructure of 30CrNiMo8 steel transformed from granular bainite mainly to martensite mainly, and the hardness also increased gradually; the critical cooling rate of martensite transformation was 1.0-1.5℃ ·s^-1, which showed 30CrNiMo8 steel had better hardenability.