Abstract: The CCT curve of 42CrMo4 steel was measured by thermal dilatometer. It was determined that the end point of austenite transformation was 823.2 ℃ and the critical cooling rate of martensite transformation was 10 ℃/s. After water-cooled,42CrMo4 steel was tempered at 570-720 ℃. The effects of tempering temperature on the microstructure and mechanical properties of 42CrMo4 steel were studied by means of microstructure observation and XRD analysis of extracted precipitates.The results show that the precipitates of 42CrMo4 steel is composed of M₈C₇ and M₃C when the tempering temperature is 570-660 ℃. The content of M₈C₇ decreases gradually with the tempering temperature increasing. When the tempering temperature exceeds 690 ℃, the precipitates is M₃C. When tempered at 570-720 ℃, the strength of 42CrMo4 steel decreases and the toughness increases with the increase of tempering temperature.