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Alloy design of a Co-Ni-Al-W-Ta-Ti senary alloy was performed through a CALPHAD (CALculation of PHAse Diagrams) approach coupled with experimental investigation, with aim to attain high γ′ solvus temperature, microstructural stability, as well as low density. The results of the thermodynamic calculations indicated that increasing the Ni content would broaden the γ + γ′ two-phase region, meanwhile simultaneously increasing the Ni content and the Al/W ratio would lead to high γ′ solvus temperature and microstructural stability. The experimental results indicated that a high Al/W ratio with a high Ni content of > 20 at.% was beneficial not only for obtaining a high γ′ solvus temperature, but also for realizing high microstructural stability and low density in these Co-Al-W-base superalloys. Increasing the Ni content enhanced the hardness of both the γ and γ′ phases, while increasing the Al/W ratio had an opposite effect. The optimized alloy, Co-30Ni-11Al-4W-1Ta-4Ti (at.%), demonstrated γ′ solvus temperature of above 1200 °C, low density of 8.76 g/cm3, and excellent microstructural stability after aging at 1150 °C for 1000 h. This study will be helpful for the continuous efforts on the development of Co-Al-W-base superalloys for industrial application, as well as populating thermodynamic databases. Keywords: Co-Al-W-base superalloys, CAHPHAD, γ′ solvus temperature, Al/W ratio, Microstructural stability