A high-pressure ac calorimetry system available between 20-200 K and 0-1.2 GPa was constructed for measuring ac heat capacities of single crystals and pellet samples with 0.5-5 mg. We used a thin film Pt chip sensor in order to cover this temperature region. We report temperature and pressure dependences of the resistance of Pt sensor which ensure the availability in high-pressure measurements. We also introduced a set of DC amplifiers and filters for minimizing noise in the signal line efficiently. Using this apparatus, thermodynamic data of Fe3O4 which shows a charge order (CO) transition accompanied by a step like change in temperature dependence of magnetization are presented. The heat capacity peak observed around 124 K was found to be broadenedunder external pressure of 1.05 GPa.