Low carbon martensitic precipitation hardening stainless steels are widely utilized in many engineering applications due to their high strength with reasonable toughness, ductility and corrosion resistance. However, those properties and their combinations are not always satisfactory to their users. For further improvement of the mechanical properties of these types of steels, a fundamental understanding of the detailed microstructural features with various aging conditions is necessary. Therefore, the effects of aging temperature, aging time and cooling rate on the microstructure and mechanical properties of a martensitic precipitation hardening stainless steel FV520B were investigated by OM, SEM, TEM and XRD methods. The results show that the steel aged at 630 degrees C for a short time and then furnace cooled, in which a typical lath martensitic with the proper amounts of reverse austenite and fined dispersed precipitates was observed, has a, good combination of high strength and high toughness. It could be an optimized aging process for FV520B steel.