Metal powder additive manufacturing (AM) technologies, such as selective laser melting (SLM), have attracted considerable interest owing to their near-net forming characteristic and layer-by-layer building-up strategy, which allows overcoming the constraints of traditional manufacturing technology, achieving complex components in a short time of mass customization. However, the SLM process-induced micro-defects (i.e. pores, lack-of-fusion, and undesired microstructures) will result in not only poor surface finish and interior thermal cracks but also more dispersion of mechanical properties. Therefore, for a more homogenized microstructure and smaller material anisotropy, a novel surface strengthening method of severe surface plastic deformation, ultrasonic nanocrystalline surface modification (UNSM) was applied to improve the surface integrity and mechanical properties of SLM 316L stainless steel in this study.