This study aimed to characterize the mechanism of thermal runaway phenomenon in lithium-ion batteries, which represent secondary cells among energy storage devices. Thermal runaway reaction was induced by heating 18650 cells with 5%, 40%, and 80% state of charge (SOC). We divided the thermal runaway of the battery into three stages and discussed the physical measurements that distinguish each stage. We also provided a visual comparison and thermal image of the characterized exhaust gases in all stages. The state of charge and the amount of heat generated by thermal runaway were proportional, and in the third stage of thermal runaway, where the highest mass transfer occurred, 40% of SOC released gas for 13 seconds and 80% of SOC emitted gas and flame for 3 seconds. In addition, a temperature and voltage measurement method that can predict the thermal runaway phenomenon of a battery is presented.