In order to characterize the heat generation behavior of a lithium-ion secondary battery during discharge, calorimetry of the battery was carried out. The temperature dependency of the heat generation was studied between 283 and 333 K. An exothermic peak and an endothermic peak were observed around 4 V, except at 333 K. These peaks are caused by a phase transition of the positive electrode material, LixCoO2, between the hexagonal and monoclinic structures, which is observed at around x = 0.5. The hear of discharge consists of mainly two factors, the battery reaction and the electrochemical polarization. In the temperature range between 283 to 333 K, if the battery is discharged at 50 mA, the least heat generation is observed at about 300 K owing to the temperature dependence of both factors.