Using Inconel 625 wire to weld high yield strength steels or stainless steels that commonly used in nuclear power plant components and gas turbines can significantly improve high temperature mechanical properties and corrosion resistance of weld structure. However, toughness, fatigue strength and creep rupture strength of weld would decline obviously because of the precipitation of δ phase during service at elevated temperatures for a long time. This work aims to investigate nucleation mechanism of δ phase in Inconel 625 deposited metal by means of SEM and TEM. Meanwhile, coarsening inherent law of δ phase during post-weld heat treatment (PWHT) at 850 ℃ for 2, 4 and 8 h respectively was revealed. The results indicate that a large number of needle-like δ phase precipitates in Inconel 625 deposited metal after PWHT at 850 ℃. These δ phases appear a grid-like distribution in γ-matrix, and there are some poor γ" phase regions appearing near δ phase. Formation process of δ phase is a solid phase transformation process which is like bainite transformation in steels. Crystal nucleus of δ phase form in the close-packed plane of γ" phase by shear mode, and coarsening behavior of δ phase is a diffusion-controlled growth process. When PWHT holding time is shorter, actual average size of δ phase is in line with LSW theory. With PWHT holding time extending, its actual average size deviates from the predicted value of classical LSW theory, because of the high-density and non-directional precipitation characteristics of δ phase.