Thermal simulation was applied to 2 Wh-class cells (diameter 14.2 nun, height 50 nun) using LiNi0.7Co0.3O2 or LiCoO2 as the positive electrode material, in order to clarify the thermal behavior of the cells during charge and discharge. The thermal simulation results for the 2 Wh-class cells showed a good agreement with measured temperature values. The heat generation of a cell using LiNi0.7Co0.3O2 was found to be much less than that using LiCoO2 during discharge. This difference was considered to be caused by the difference in the change of entropy. A 250 Wh-class cell (diameter 64 mm, height 296 mm) was also constructed using LiNi0.7Co0.3O2, and thermal simulation was applied. We confirmed that the results of the thermal simulation agreed with measured values and that this simulation model is effective for analyzing the thermal behavior of large-scale lithium secondary batteries,