The heat capacity of polytetrafluoroethylene (PTFE) crystal around the phase transition temperatures have been measured with a new type of high pressure ac calorimeter developed in the authors’ laboratory. A temperature-pressure phase diagram was proposed. The phase II undergoes the phase transitions on the heating process to the phase IV (0.1 - ca. 200 MPa), the phase I (ca. 200 - ca. 400 MPa), the phase I’ (ca. 400 - ca. 500 MPa) and the phase III (higher than ca 500MPa). At pressures lower than 200 MPa the phase IV further changes to the phase I. Based on the temperature dependence of the heat capacity the phase transitions from the phase II to the phases IV, I and I’ were attributed to the order-disorder type. The structure of the phase diagram suggested that the phase I and the phase I’ were thermodynamically close to each other. This was consistent with the vanishing of the boundary between the phase I and phase I’ in the middle of the region of the phase I. The temperature dependence of the heat capacity around the phase transition to the phase III was notably different from the transitions to the phase IV, I and I’. Nature of the phase III was discussed in comparison with the high pressure phase of polyethylene.