Abstract: By means of microstructure observation, impact experiment and hardness measurements, the effects of different tempering processes on the microstructure and mechanical properties of 8Cr4Mo4V bearing steel were studied.Results show that, under the experimental conditions, the microstructure of 8Cr4Mo4V bearing steel is mainly composed of bainite, martensite, retained austenite and carbides.The following three tempering processes were carried out.A:550 ℃×2 h(3 times), B:350 ℃×2 h(1 time)+350 ℃×2 h(1 time)+550 ℃×2 h(2 times), and C:350 ℃×2 h(2 times)+550 ℃×2 h (2 times).The impact energy is 13.1 J, 8.75 J and 8.52 J respectively for the steel with the tempering treatments A, B and C.Temper brittleness occurs to the samples with B, C tempering treatment, which reduces the toughness of the steel.The hardness of the steel with the tempering treatments A, B and C is 60.9 HRC, 62.3 HRC and 62.5 HRC, respectively.Compared with the tempering treatment A, the microstructure of the steel with the tempering treatments B and C has more carbides, and the dispersion strengthening effect is stronger, so the hardness of the experinent steel is enhanced.