Recently, the number of fire accidents of mobile products using lithium-ion batteries has increased year by year. Overcharge is one of serious causes of thermal runaway for lithium-ion batteries resulting in firing, which occurs in abuse or by malfunction of charging equipment. In order to prevent thermal runaway of lithium-ion batteries in the case of overcharge, appropriate thermal management is required with accurate knowledge for heat generation mechanism of lithium-ion batteries under such accident, and calorimetry is an effective method to characterize thermal behaviors of lithium-ion batteries. In this article, some results of calorimetric analysis of commercially available lithium-ion batteries during overcharging are introduced, where the heat generation is practically proportional to the charging current and almost equivalent to the charging electric power. The electrolytic decomposition reaction of the organic solvents with gas generation is the main reaction at the positive electrode in overcharge. It is noteworthy that even in standard charging condition, a similar exothermic reaction proceeds as like overcharging in some degraded batteries, and this might be one of reasons of thermal runaway.