The purpose of this paper is to comprehensively improve the mechanical properties of SLMed Inconel 718 alloy in different building directions by studying the influence of hot isostatic pressure treatment on the microstructure and mechanical properties in the parallel and perpendicular to building direction. Inconel 718 alloy samples were prepared by selective laser melting technology, and SLMed samples were treated by hot isostatic pressure and solid solution aging heat treatment. The microstructure, fracture morphology, phase composition, grain morphology and orientation of the alloy were analyzed by means of metallographic microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffractometer (XRD) and electron backscattering diffraction (EBSD). The microstructure hardness, tensile strength, yield strength and elongation rate of different forming direction of SLM formed and hip treatment and HIP+HT treatment were analyzed. The results indicated that the grain morphology of SLM Inconel 718 alloy along the building direction had changed from columnar to equiaxial crystal, and the grain size had increased, accompanied by twin formation after the heat isostatic treatment. Meanwhile, most Laves phase around grain boundary had disappeared, and many MC carbides were precipitated in γ matrix. The strength and hardness of the parallel to the building direction were greater than those perpendicular to building direction after different treatment. The tensile strength of the SLMed Inconel 718 alloy parallel and perpendicular to the building direction was 996.3 MPa and 1051.1 MPa respectively. After HIP treatment, the tensile strength in parallel and perpendicular to the deposition direction was increased to 1151.5 MPa and 1167.6 MPa, respectively. Compared with SLM and HIP specimens, HIP+HT specimens perpendicular to the building direction exhibited the highest tensile properties, which was superior to the technical standard values of forgings. The fracture analysis shows that the fracture is ductile in three conditions. Hot isostatic treatment can change grain morphology and orientation in different directions, eliminate cracks and pores and other defects, so the mechanical properties of Inconel 718 alloy fabricated by SLM can be improved comprehensively.