In this study, the grain refinement near the surface of metal workpiece using hot shot peening was investigated to improve the surface properties of the workpiece. In this process, the grains were refined due to plastic deformation generated by the collision of a lot of shots under hot working conditions. A model experiment using two shots was carried out to examine the effects of the amount of deformation, the processing temperature and the time interval of the collision on grain size. In the experiment, the workpieces were stainless steel SUS304 and commercially pure copper. It was found that the global surface layer successfully attained to the fine grains by means of hot shot peening.