Fe20Ni3Mn alloy is treated by LSP. The hardness, residual stress and microstructure in the strengthened zone are studied. The results show that the hardness of the strengthened surface is increased about 15% and the surface compressive residual stress is reached about -469MPa. There exist martensite transformation and dislocation within 1mm depth from the specimen surface. Some martensitic structures have obvious trend of growth in steps. Specially, the twin crystals in martersite induced by LSP have important application.