Abstract: 12Mn1CrMo low alloy steel was subjected to oil quenching at 850 ℃for 30 min, then the tempering tests at 260-580 ℃were carried out.The effects of tempering at different temperatures on the microstructure and mechanical properties of 12Mn1CrMo steel were studied by OM, SEM, tensile test, impact test and impact fracture observation.The results show that the microstructure of quenched 12Mn1CrMo steel is fine lath martensite.With the increase of tempering temperature, the martensite decomposition degree in 12Mn1CrMo steel is higher and higher.The martensite is completely decomposed in the 580 ℃tempering 12Mn1CrMo steel, and the microsstructure is tempered sortensite.With the increase of tempering temperature, the tensile strength of 12Mn1CrMo steel decreases gradually, while the elongation and impact toughness decrease first and then increase.The maximum elongation and impact toughness of 12Mn1CrMo steel tempered at 580 ℃are 13.8%and 131 J/cm², respectively.