Phase diagram of Co-Ge binary system was thermodynamically assessed by using CALPHAD approach in this study. The excess Gibbs energy of the solution phases, liquid, alpha(Co) and epsilon(Co), were modeled with Redlich-Kister polynomial. Magnetic contribution to the Gibbs energy was also taken into account for alpha(Co) and epsilon(Co). Considering its crystal structure and solubility range, the intermetallic compound beta Co5Ge3, with B8(2)-structure, was particularly described with a three-sublattice model, (Co,Va)(1):(Co)(4):(Co,Ge)(3). And the compound CoGe was described with two-sublattice model according to its crystal structure. Other intermetallic compounds were described as stoichiometric phases because of their narrow homogeneity ranges or unknown crystal structure. In order to obtain a reasonable description of several Co-Ge compounds, first-principle calculations were performed before optimization to determine their formation enthalpies. Finally, a set of thermodynamic parameters was finally obtained so that most data of phase boundaries and thermodynamic properties of various phases were reproduced in present optimization. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.