Molybdenum (Mo) addition was investigated for its influence on the interfacial characteristics of γ′-phase strengthened Co-based superalloys with γ/γ′ dual phases. Advanced characterization techniques, including scanning transmission electron microscopy (STEM), atom probe tomography (APT), and high-resolution X-ray diffraction (HR-XRD), were employed for this purpose. Mo addition was found to increase the compositional and structural widths of the γ/γ′ interface. This was accompanied by segregation of Mo and Co at the interface, attributed to the reduction of interfacial energy. Additionally, Mo addition led to a decrease in the lattice misfit between the γ-matrix and γ′-phases. Those results enhanced interface stability, contributing to improved high-temperature stability. This study offers valuable insights into the mechanisms of alloying and the microstructural stability of Co-based superalloys.