By using high-energy shot peening (HESP) techniques, nano-structured surface layers were successfully created on some metallic materials. The distribution of defects in the surface layer was studied by means of positron lifetime spectroscopy for samples treated with different processing times. We determined the positron lifetime characteristics as a function of depth from the top surface. The results showed two microstructure regions on the surface layer: an ultra-fine grain region and a sequential transition region.