Superalloy Inconel 718 is an important material used for aero–engine high temperature turbine disks. The grain refining of Inconel 718 becomes critical because of the improvement in the quality and reliability of aero–engine. Inconel 718 turbine disks are manufactured by multi–stage hot deformation processes, in which the recrystallized grain grows up in next passes. Therefore, it is difficult to obtain a uniform and refined microstructure by recrystallization refining. The δ phase in Inconel 718 can control grain size through the strong pinning effect. Thus, the Delta process (DP) has been applied for the forging of Inconel 718. In this paper, for the DP of Inconel 718, the evolution of δ phase during isothermal compression deformation at temperature of 950℃ and strain rate of 0.005 s⁻¹, was studied by using optical microscope (OM), scanning electron microscope (SEM) and quantitative X–ray diffraction (XRD) technique. The results show that spherical or rod–shaped δ phase particles in the interior of grains precipitated in the aging treatment disappear during the heating and holding time before deformation, and thcontent of δ phase decreases from 8.14% to 7.05%. Dissolution of δ phase occurs during the deformation, and the content of phase decreases from 7.05% to 5.14%. The spheroidization of plate–like or needl–like δ phase takes place due to the effect of deformation and dissolution breakages, and the plate–like or needle-like δ phase transferrs to sphrical or rod–shaped δ phase. In the centre with the largest strain, the plate–like or needle–like δ phase disappears and spherical or rod–shaped δ hase appears in the interior of grains and grain boundais.