The effect of heat treatment on the microstructure and mechanical properties of aged hardening 2Cr16Ni2MoN stainless steel was examined. The steel contained undissolved (CrFeMo)23C6 carbides in a martensite at lower solution treated temperature. Solution treating at 1100°C resulted in coarsening of the packet and block structure of the lath martensite. The optimum solution treated temperature is 1040°C.Aging at 500°C resulted in peak strength due to the precipitation of extremely fine coherent (CrFeMo)2C carbides. The optimum combination of tensile strength and impact toughness was attained at the aging temperature of 560°C,was attributed to both the formation of fine M2C in the lath and a mixture of lath martensite with high density dislocations.