Characteristics of thermal runaway of lithium-ion battery are analyzed by considering the internal reactions occur in battery cell. We developed battery thermal runaway model to analyze relationship between internal reaction and temperature changes in the battery. Firstly, we analyzed temperature changes in the cell locally during thermal runaway of battery. The temperature of the battery moderately increases at the beginning, then drastically increase up to almost 600 °C. After reaching peak point, temperature gradually decreased. The temperature change is resulted from reaction heat occurs in the battery cell. Moderate temperature increase is due to reaction heat of electrolyte. However, when the temperature reaches over 120 °C, component of battery, such as solid electrolyte interface and electrolyte start to decompose. Therefore, extreme heat is generated. Next, temperature distribution inside the battery cell is examined. The temperature near tab shows the lowest value inside the battery cell. The maximum temperature near tab region is about 150 °C lower than other regions. This is resulted from the thermal conductivity of battery components. Electrode tabs have higher thermal conductivity than other components, such as electrolyte and membrane. Therefore, large amount of heat is emitted through the tab. Therefore, temperature of battery near tab region shows the lowest value during thermal runaway. This study will help understand internal phenomena in the battery during thermal runaway of lithium-ion battery.