The phase transformation and mechanical properties of two steels of ZG04Cr13Ni4Mo with 12% Cr and 13% Cr, which being subjected to 893 K one-stage tempering and 893 K + 863 K two-stage tempering respectively, were investigated by means of thermal dilatometer, XRD and tensile tester, in order to clarify the effect of Mo and Cr, the two key alloying elements in ZG04Cr13Ni4Mo martensitic stainless steel, a candidate material used widely for hydraulic turbine runners. The results show that with the increase in Mo and Cr content, the A_s temperature (the beginning temperature of austenitization) decreases gradually. For steels with different Mo contents, the amount of martensite converted into reversed austenite in the heating and isothermally holding stage may definitely different with that by directly cooling down to ambient temperature after tempering, when they are subjected to primary tempering at 893 K within the intercritical temperature zone. Therefore, due to such difference in microstructure evolution, when Mo content increases from ~0.3% to ~0.6%, the yield strength of ZG04Cr13Ni4Mo steel containing 12% Cr decreases slightly, while the ultimate tensile strength increases slightly; in the contrast, the yield strength and tensile strength of ZG04Cr13Ni4Mo steel containing 13% Cr all slightly increased after 893K tempering with the same change in Mo content.