Peening is mainly used as a method of surface treatment for microstructural modification in order to improve surface mechanical properties. The ultrasonic shot peening (USP) technique can cause severe plastic deformation with its high strain rate on the surface of metallic parts. However, systematic studies of microstructural refinement mechanism upon plastic deformation with consideration of alloy systems are rare. In this study, USP-treated Cu samples of 99.96% purity was examined using analytical techniques, Vickers microhardness test, electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). Results of EBSD and microhardness analyses indicated grain refinement with deformation structures and hardness increase down to 400 μm in depth upon treatment. Depth specific TEM analysis of the samples revealed the steps of the grain refinement process to the formation of randomly oriented fine grains.