Abstract: The effects of aging treatment at different temperatures and time on the microstructure and hardness of0Cr15Ni5Cu4Nb die steel were studied by means of microstructure observation and hardness test. The results show that the microstructure of 0Cr15Ni5Cu4Nb steel is mainly lath martensite and granular copper rich precipitates after solution treatment at 1050 ℃×1 h and aging treatment at different temperatures for 6 h. In the range of 320-460 ℃, with the increases of aging temperature, the number and size of copper rich precipitates increases gradually. When the aging temperature continues to increase to 520 ℃ and 580 ℃, the martensite in the microstructure decomposes. With the increase of aging temperature and aging time, the hardness of 0Cr15Ni5Cu4Nb steel first increases and then decreases. The hardness of 0Cr15Ni5Cu4Nb steel treated with 1050 ℃×1h solution+460 ℃×6h aging treatment reaches the maximum of 43.5 HRC.