The thermodynamic assessment of the Co-Nb-W ternary system has been carried out by applying the CALPHAD method. A slight modification of the Co-W binary system has been performed to unify the model of μ phase in Co-Nb-W system. The three published isothermal sections at 1273, 1373 and 1473 K have been critically examined and used to optimize the model parameters. The solution phases, including αCo and BCC were modeled as substitutional solutions; the λ, χ, and Co3Nb phases have been assessed by using two-sublattice models. The Co7W6 and Co7Nb6 phases were described as (Co, Nb)1(Co, Nb, W)2(Co, Nb, W)4(Co)6. Calculated isothermal sections and vertical sections agreed well with the experimental data.