Phase relations in the C–Fe–Zr ternary system were investigated using the experimental data obtained through the combination of X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA) techniques. Isothermal sections of the C–Fe–Zr system were experimentally determined at 1173 K, 1273 K, 1373 K, and 1473 K, and no ternary compound was found in this system. A self-consistent set of thermodynamic parameters of the C–Fe–Zr system were obtained for the first time using the CALculation of PHAse Diagram (CALPHAD) method, and the calculated results showed good consistency with the experimental data. It can provide reliable thermodynamic information of molten corium composition as one of necessary subsystems.