To explain the precipitation mechanism of χ phase in Co-based superalloys, the microstructural evolution of Co-Ti-Mo superalloys subjected to aging was investigated by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that the needle-like χ phase is mainly composed of D0₁₉-Co₃(Ti,Mo), which is transformed from L1₂-γ′ phase, and a specific orientation relationship exists between them. χ phase is nucleated through the shearing of γ′ phase due to the influence of stacking fault. The crystal orientation relationship between L1₂ and D0₁₉ can be confirmed as {111}_L12//{0001}_D019, and á112？ _L12//á1 00？ _D019. The growth of D0₁₉-χ phase depends on the diffusions of Ti and Mo, and consumes a large number of elements. This progress leads to the appearance of γ′ precipitation depletion zone (PDZ) around D0₁₉-χ phase. The addition of Ni improves the stability of L1₂-γ′ phase and the mechanical properties of Co-based superalloys.