To evaluate the development of the microstructure and mechanical properties on surface modified and post-heat-treated Inconel 718 alloy, this study was carried out. A friction stir process as a surface modification method was employed,and overlap welded Inconel 718 alloy as an experimental material was selected. The friction stir process was carried out ata tool rotation speed of 200 rpm and tool down force of 19.6-39.2kN; post-heat-treatment with two steps was carried out at720oC for 8h and 620oC for 6h in vacuum. To prevent the surface oxidation of the specimen, the method of using argongas as shielding was utilized during the friction stir process. As a result, applying the friction stir process was effective todevelop the grain refinement accompanied by dynamic recrystallization, which resulted in enhanced mechanical properties ascompared to the overlap welded material. Furthermore, the post-heat-treatment after the friction stir process accelerated theformation of precipitates, such as gamma prime (γ') and MC carbides, which led to the significant improvement of mechanicalproperties. Consequently, the microhardness, yield, and tensile strengths of the post-heat-treated material were increased morethan 110%, 124% and 85%, respectively, relative to the overlap welded material. This study systematically examined therelationship between precipitates and mechanical properties.