The amount of retained austenite formed during tempering process can moderately improve the structure and properties of martensitic stainless steels. In the present study, 25 heat treatment cycles were carried out on specimens cast in CA6NM in order to investigate the effect of tempering treatment on the structure and mechanical properties of this alloy. X-Ray diffraction, mechanical testing, optical and scanning electron microscopy were employed for evaluation of the specimens. SEM micrographs and X-ray diffraction patterns indicated that with enhancement of tempering temperature, the amount of retained austenite initially increased and then decreased. The changes in mechanical properties of specimens including hardness, impact toughness, elongation, yield and tensile strength were found to depend mainly on the amount of retained austenite. Such variations are discussed in terms of amount of retained austenite and its stability during tempering process of martensite.