Lithium ion battery is widely used in electronic products with the advantages of its high specific energy, high theoretical capacity and long span life. The thermal runaway of lithium ion battery is rare in probability, but the thermal runaway caused fires and explosions always occur with the huge used numbers. It is necessary to find how the thermal runaway developing in a single battery. The thermal runaway processes under external heating mode and internal heating mode were investigated in an extended volume accelerating rate calorimeter (EV-ARC) in this work, respectively. The batteries were charged to 0, 25, 50, 75 and 100% SOC, and then they were put into a 150 °C cavity to simulate the external heat conditions, respectively. The results show that the time to thermal runaway decrease with the SOC increasing. The discharging was used to simulate the internal heat influence on the thermal runaway, and the discharging rates were set from 1 A to 5 A with 1 A interval. The results show that the normal cycle number decreases with the current increasing before the thermal runaway, and the time to thermal runaway is shortened. The thermal runaway always occurs at the end of constant voltage charge stage during internal heat experiments. The experimental results indicate that the SOC plays an important role in the process of thermal runaway. The internal exothermal reactions process of battery during thermal runaway could be summarized as the SEI decomposition, anode reaction consumption of Li, separator melting, reactions between internal materials, and then inner short circuit, followed by temperature rising rapidly.