Abstract: Two single crystal superalloys with 1 % Mo and 3 % Mo were prepared by vacuum directional solidification furnace to optimize the Mo content of the fourth-generation single crystal superalloy. Phase diagrams of the alloys with different Mo contents were calculated by JMatPro software. The effects of Mo contents on the solidification characteristics, as-cast microstructure, heat treatment microstructure and long-term aging microstructure at 1 100 ℃/1 000 h were investigated by SEM and EDS. The results show that with the increase of Mo content, the solidus and liquidus temperature decreases, the mushy zone becomes wider, and the volume fraction of γ phase and TCP phase increase. The volume fraction of γ-γ′eutectic decreases, and the size of γ′ phase decreases slightly and their homogenization and regularity have been improved. Moreover, when the cubic shape of γ′ phase becomes more regular in the heat treatment microstructure, the volume fraction of TCP phase significantly increases and the microstructure stability decreases.