Stainless steels have good corrosion resistance and mechanical properties. The stainless steels have a lot of application in industries such as ball bearings, surgical instruments, turbine blades and manufacturing equipment such as tanks or pipelines. Using steel in industrial applications requires proper selection of heat treatment parameters. One of the martensitic stainless steel is the AISI431 steel. Due to martensitic stainless steels are more sensitive to heat treatment cycles, so, the cycles of heat treatment of the steel must be properly identified. One of the phases formed in the micro structure of the steel after austenitising and quenching is Delta-ferrite. The amount of this phase affects on the mechanical properties of the steel. In this study, tensile and impact specimens at temperatures of 950°C, 1050°C, 1150°C and 1250°C are hold in for 1, 2 and 3 hours, then quenched in oil. Double tempering treatment at 660°C and 650°C have been done for two hours. Effect of heat treatment parameters on mechanical properties and microstructure of the steel AISI431 was studied. Using metallography and image-j software, the amounts of delta-ferrite has determined. The show that The minimum amount of delta-ferrite was obtained for heat treated sample at 1050°C, for two hours and the highest tensile strength and impact energy were obtained respectively for heat treated samples at 1250°C and 1050°C , for one and two hours. It was also shown that delta ferrite causes degradation of the mechanical properties of steel. The only positive effect of delta ferrite on the mechanical properties, increasing the yield stress. Key words : martensitic stainless steel AISI 431, heat treatment, delta-ferrite, mechanical properties.