Based on the assessed three constituent binary systems and reported phase diagram data, the Co-Al-W ternary system was thermodynamically optimized by using CALculation PHAse Diagram (CALPHAD) method. The newly reported ternary phase gamma' was described with a two sublattice model, (Co,Al,W)(0.75):(Co,Al,W)(0.25), while liquid Fcc, Hcp(Co), and Bcc(W) were treated as substitutional solution phases. In order to describe the ternary solubility in the binary phases Co7W6 and CoAl, the models (Al,Co,W)(7)W-2(Al,Co,W)(4) and (Al,Co,W)(0.5)(Al,Co,W)(0.5)(Va)(3) were, respectively, adopted. The rest 11 phases in the ternary systems were simply treated as stoichiometric compounds because their homogeneous ranges are small and can be neglected. A set of self-consistent parameters were obtained, which reasonably reproduced the phase relationship in Co-enriched corner. Finally, to confirm the reliability of the present assessment,the so-called Scheil-Gulliver model was used to simulate the solidification processes of three typical Co-based alloys. It was shown that the as-cast microstructures of the three ingots were well explained or predicted.