Cast alloy IN625 is a candidate Ni-based alloy for advanced ultra-supercritical (A-USC) power plants. In order to investigate the influence of Mo on δ phase precipitation, the heat treatment experiments on the Mo series of alloys were carried out at 750 • C (the service temperature of A-USC power plants) up to 17,000 h, and effects of δ phase on the tensile properties were also studied. The results show that lots of δ phase precipitated after longterm aging at 750 • C and the Mo content has a weak effect on the precipitation of the δ phase. With the increasement of δ phase content, the strength of alloys after aging at 750 • C for up to 5,000 h increased nearly linearly with the increasing aging time while the elongation was affected by δ precipitation to a relatively limited extent. It is suggested that the δ phase serves as a strengthening phase in cast alloy IN625.