The work aims to investigate the effects of heat treatment on the microstructure and corrosion properties of 18Ni300 maraging steel fabricated by Selective Laser Melting (SLM). 18Ni300 maraging steel samples were fabricated by selective laser melting and aging treatment and solution-aging treatment were respectively conducted to the SLM samples. The microstructure, microhardness and corrosion properties of the SLM 18Ni300 maraging steel samples under different heat treatment were studied by means of metallographic microscope, scanning electron microscope, hardness tester and elec-trochemical workstation. It was found that the microstructure of these samples changed significantly after heat treatment. The microstructure of the aged sample was refined and composed of lath martensites. The molten pools of the solution-aged sample disappeared, and the microstructure was composed of dense uniform lath martensites. Fine precipitates were distributed between the grain boundaries and the laths in the heat treatment samples. Both aging treatment and solution-aging treatment improved the hardness of SLM 18Ni300 maraging steel. The hardness of the SLM sample was 376.6HV1, and the hardness of the aged sample and the solution-aged sample were 651.5HV1 and 641.8HV1, respectively. In 0.5 mol/L H2SO4 solution, the SLM sample had the best corrosion resistance, as its Jcorr was the smallest (1.375×10-3 A/cm2). The corrosion resistance in 0.5 mol/L H2SO4 solution was: the SLM sample>the solution-aged sample>the aged sample. In 3.5% NaCl solution, the SLM sample had the smallest Jcorr (3.630×10-6 A/cm2), and an obvious passivation platform appeared in its polarization curve, so its corrosion resistance was the best. The corrosion resistance in 3.5% NaCl solution was: the SLM sample>the aged sample>the solution- aged sample. After aging treatment and solution-aging treatment, the microstructure of SLM 18Ni300 maraging steel is lath martensite, and the hardness increases significantly, but the corrosion resistance in H2SO4 solution and NaCl solution decrease.