Various heat treatments were conducted on Inconel 718 superalloy, and the resultant microstructures and properties were investigated to analyze the mechanisms of heat treatments. Results show that the type and quantity of precipitate phases and the grain size have different effects on the properties of Inconel 718 superalloy after various heat treatments. The γ" and γ' phases as well as grain size mainly influence the strength, and the δ phase mainly influences the plasticity. Besides, the precipitation of γ" and γ' strengthening phases can improve the yield strength. The alloy strength is inversely proportional to mean grain size when the γ" and γ' phases have similar contents. The plasticity is susceptible to the content and shape of δ phase. A proper amount of δ phase is beneficial to the plasticity, but excessive δ phase degrades plasticity.