Abstract: Selective laser melting (SLM) is a non-equilibrium solidification process and can cause the thermal and structural stress, resulting in the cracking and deformation of metal parts. Post-heat treatment is an important way to improve the microstructures and properties of SLM metal parts. The effects of post-heat treatment parameters on microstructures and properties of S136 die steel formed by SLM were investigated in this paper, including heat treatment temperature, holding time, and cooling mode. In the results, the optimal heat treatment parameters are found as 1050℃, 30 min, and water cooling. Rockwell hardness is found up to HRC 53.7, increasing by 8.6% than that of as-formed sample. The uniformity of hardness is greatly improved as the standard deviation of Rockwell hardness decreased by 37.14%. The wear resistance, maximum micro-hardness, and maximum elastic modulus of heat treated sample are increased by 33.5%, 3.3% and 8.6%, respectively. The scaly structure morphology of molten pool is disappeared, CrFe₇C_0.45 carbide is dissolved, and the uniform and isotropic microstructures are present.