The heat generated in the operation of UPS system is dissipated by cooling fans. Compared with natural convection cooling, the cooling effect of fan is better, and the temperature in the system can be reduced up to 30 ℃. The expected life of electronic components requires extensive modeling, laboratory testing and field verification. In UPS system, if the fan fails, the velocity of air flow used for forced cooling air will decrease, and the temperature of components in the system will rise. In order to achieve the expected life of UPS system, periodic maintenance and replacement of cooling fans are needed. This paper briefly discusses the effect of temperature on insulation aging, the difference between natural convection and forced convection, the reliability modeling of cooling fans and how periodic maintenance improve the reliability of UPS system.