Abstract: Inconel 718 is the most popular nickel-based superalloy and is extensively used in aerospace, automotive, and energy industries owing to its extraordinary thermomechanical properties. The effects of different two-step solid solution treatments on microstructure and δ phase precipitation of Inconel 718 alloy were studied, and the transformation mechanism from γ″ metastable phase to δ phase was clarified. The precipitates were statistically analyzed by X-ray diffractometry. The results show that the δ phase content firstly increased, and then decreased with the temperature of the second-step solid solution. The changes in microstructure and δ phase were studied by scanning electron microscopy and transmission electron microscopy. An intragranular δ phase formed in Inconel 718 alloy at the second-step solid solution temperature of 925°C, and its orientation relationship with γ matrix was determined as \bar 100_\text δ //01\bar 1_\text γ and (010)_δ//(111)_γ. Furthermore, the Vickers hardness of different heat treatment samples was measured, and the sample treated by second-step solid solution at 1010°C reached the maximum hardness of HV 446.84.