Transport equipment is being made lighter and stronger, and surface treatment is being actively carried out to improve surface properties. Shot peening is known as one of the surface treatment techniques. A work-hardened layer and compressive residual stress are applied to the surface by colliding small steel balls against the metal surface. This technology is expected to improve the fatigue strength and stress corrosion cracking resistance of machine parts. In the present study, the effect of microshot peening (MSP) on the tensile properties of austenitic stainless steel treated with water from high temperature was investigated. Austenitic stainless steel has been widely used as a member of chemical plants and buildings for a long time because of its excellent mechanical properties, workability, and heat resistance in addition to corrosion resistance. The test material was a commercial austenitic stainless steel SUS304 rolled sheet. Tensile test workpieces were cut from the rolled sheet to half size of JIS No. 13 B type. Heat treatment was carried out in the temperature range of 400°C to 1100°C, and water quenching was performed after holding for 1 hour. MSP was applied to the stainless steel after heat treatment and evaluated by a tensile test. A rapid increase in tensile strength and a decrease in elongation at break were observed in the heat treatment temperature range of 600–900°C. In addition, MSP increased the tensile strength.