This study evaluated the mechanical properties of friction stir welded and post-heat-treated Inconel 625 alloy. Friction stir welding (FSW) was performed at rotation and traveling speeds of 200rpm and 100mm/min, respectively; heat treatment was carried out after welding at 700°C for 100h in vacuum. As a result, the application of FSW on Inconel 625 alloy led to the grain refinement in the stir zone, which resulted in increase in mechanical properties than those of the base material. Especially, applying heat treatment after FSW led to the improvement of mechanical properties of the welds; microhardness and tensile strength increased by more than 30% and 50%, respectively, as compared to FSW alone.