In Taiwan, there is a fire accident occurred in the battery storage room of a thermal power plant. After field observation and preliminary investigation, the main failure was happened in a lead acid battery's positive terminal, which was seriously burned. Due to the use of the battery in the fire burned never encountered before, it is worth to further clarify the failure cause. In this study, background information, macro observation, failure cross section investigation, metallographic observation of failure material, corrosion products analysis were used to investigate the failure cause of the lead acid battery. According to the results of this study, corrosion is the main failure cause, where the other influencing factors including material's defects, electrode oxidation reaction, design of battery's electrode plates and environment temperature. The estimated failure mechanism is proposed, the electrical conducting area of battery's positive terminal was reduced due to corrosion, which caused electrical resistance increased. After cyclic charge and discharge, the current became too high for the battery, which leaded to a high temperature and caused the battery being burn out.