The calculations of phase equilibria of the Co-Cr-Pt ternary system have been carried out based on the thermodynamic assessments of Co-Cr, Co-Pt and Cr-Pt binary systems by the calculation of phase diagram technique. The Gibbs energies of the liquid, FCC, BCC and HCP solution phases were approximated by a sub-regular solution model, while those of σ and Cr3Pt phases were approximated by a compound energy model. Almost all the experimental information on each sub-system has been well described by the present set of thermodynamic parameters. A critical calculation of the magnetically induced miscibility gap between the ferromagnetic HCP and the paramagnetic HCP phase has been conducted, where the two-phase separation has been found at the Curie temperature. The Cr content in the ferromagnetic HCP phase increases and the width of the two-phase separation becomes narrower with increasing Pt content. The present calculations would be useful for the design and development of the perpendicular magnetic recording media.