The Co-Cu-Sn ternary system has been modeled based on reported phase equilibrium data in the literature using the CALPHAD (CALculation of PHAse Diagrams) method. The excess Gibbs energies of solution phases, including liquid, Bcc, Fcc and Hcp, are expressed by the Redlich-Kister polynomial. The two-sublattice model (Co,Cu)_m(Sn)_n is used to describe the solid solution of binary intermetallic compounds, i.e. CoSn₃, CoSn₂ Cu₃Sn and βCu₆Sn₅ in the Co-Cu-Sn ternary system. Co₃Sn₂ was described using the four-sublattice model (Co,Cu,Sn)₁(Cu,Sn)₁(Co,Va)_0.5(Co,Va)_0.5. The ternary stoichiometric compound Co₂Cu₈Sn₃ is modeled by the stoichiometric model, Co₂Cu_7.5Sn₃. Finally, a set of self-consistent parameters which can describe the thermodynamics of the Co-Cu-Sn ternary system was obtained. Based on the calculated thermodynamic parameters, the liquidus projection and reaction scheme are also derived in the present work.