To make clear the effect of fine-particle-peening (FPP) on the plastic deformation of materials, tensile deformation and hardness measurement tests have been performed for SP and FPP treated steels. As a result, the ductility decreases for SP treated steel, but increases that for FPP treated steel. From a change of hardness distribution of inside the steel inside during tensile deformation processes, near the surface region of FPP treated steel has more deep stable hardened layer than SP treated steel for tension deformation. It seems to be a cause of increase of a ductility of FPP treated steel that hardness distribution becomes more heterogeneous and having more deep stable hardened layer during tensile deformation.