Selective laser melting（SLM） has unique advantages in the complex metal components manufacturing field. The type and the formation mechanism of the pores in the SLM processed 316L SS samples were analyzed,providing theoretical basis for the defect control. The experimental results show that the defects of internal pores in the SLM processed 316L SS mainly include: triangle pores at the molten pool junction,randomly distributed pores originating from inclu-sions,irregular shallow pores in the center of the molten pool and aggregative distributed pores originating from incomplete fusion of powder. The defects of surface pores mainly include the shallow pitting defect and the deep defect characterized by a thin neck. There are inclusions inside the defect and microcracks are easily produced at the tip of the surface pores.The spatters produced in SLM process are the main cause of internal and surface pores,and the analysis results show that the spatters are mixed oxidation products at high temperature.