Recently, prevailing Lithium ion batteries(LiBs), mounted on electric vehicles have the risk of exploding by a fire caused by a collision with a gasoline car. The purpose of this study is to investigate whether commercial LiBs caused fire by external heating, and provide recommendations on the safety of LiBs. In this experiment, we heated the bottom of LiBs which were contained in an aluminum box by a 300 W heater for one hour, to make the battery in an unstable state electrochemically, thereby causing the battery to catch fire by internal short circuit. Also determined was battery voltage, internal battery temperature and heater temperature to observe the voltage behavior to battery temperature while heating. As a result, when the temperature of the battery reached about 80°C, the safety function of the battery operated, and the current was broken automatically. However, a battery fire occurred, which was caused by internal short circuit when the battery temperature reached about 140°C. Therefore, this experiment confirmed that only the safety function mounted on LiBs were unable to prevent battery fire. Moreover, when electric energy remains inside of the LiBs, we confirmed the energy must be released because there is a risk of the battery catching fire to occur again, even after extinguishing it.