CO2 laser welding characteristics of Inconel 718, heat treatment effects on microstructures, and tensile/fatigue properties were researched. CO2 laser welding was carried out on 5 mm thick plates having two different grain sizes, ASTM #4 and #10. For the fine-grain sized specimens (ASTM #10), the optimum laser welding conditions without defects were 6 kW with 2.5 m/min and 8 kW with 4.0 m/min. However, on the heat-affected zone (HAZ) of large-grained Inconel 718 (ASTM #4), the microfissures were shown at the same welding conditions. Before welding, plates were pre-heat treated at three different conditions: as-received, 955 deg C solution treatment, and 955 deg C solution treatment + aging (955STA). After welding, welds were subjected to four different post-weld heat treatment conditions: as-welded, 955 deg C solution treatment + aging, modified cyclic solution treatment, and direct aging (DA) treatment. In this research, modified cyclic solution heat treatment showed reduction of microfissures and improved tensile and fatigue properties.