Abstract: Microstructure and hardness of 4Cr5MoSiV steel quenching and tempering have been observed and analysed by metallographic microscope and the hardness tester. The result show that, carbides of 1 040℃, 1 060℃ quenching were not completely dissolved in austenite, martensite microstructure becomed thicker increases with the quenching temperature; Hardness increased first and then decreased with temperature raised, maximumvalue was 56.5 HRC at 1 100℃; Grain size decreased gradually with the increased of temperature. The solubility of carbides in autenite had a great effect on the hardness of martensite transformation. Tempering was at the range of 490℃-560℃ after 1050℃ and 1 100℃ quenching, obtained different microstructure, and many types of carbides were precipitated in the tempering microstructure, mainly including M₂₃C₆ and M₆C, tempering hardness wasobviously improved by those carbides of microstructure.