In this work, several thermodynamic assessments adopted widely for ZrO2-CaO system are reviewed and examined, the existing discrepancies are summarized, and a new assessment is carried out based on the formation enthalpy of two compounds (CaZr4O9 and Ca6Zr19O44) and experimental activity data concerning cubic solid solution. The thermodynamic parameters of all phases have been optimized by the least squares minimization procedure, a self consistent set of the optimized Gibbs energy parameters has been derived, which can be safely used to extrapolate into the multicomponent system. Compared with experimental data and the results in this work as well as the results reported previously, it is demonstrated that the present thermodynamic assessment is in better agreement with most of the experiments.