Abstract: Cr5Mo1V die steel specimens were air quenched at 950 ℃and then tempered at 520 ℃by different time(30 min, 60 min, 120 min and 180 min).The microstructure evolution and hardness changes of Cr5Mo1V die steel under different holding time were analyzed by optical microscope, scanning electron microscope and hardness tester. The results show that with the increase of tempering time, the microstructure of Cr5Mo1V die steel changes from tempering martensite to tempering sorbite. With the extension of holding time, the hardness increases first and then decreases, and reaches the maximum value after 120 min tempering.Therefore, the best choice of tempering time for Cr5Mo1V die steel is 120 min.