A process for controlling temperature of a battery pack includes operating a fan to change a battery temperature to an optimal range of temperature (T.sub.opt) when a condition is satisfied that the battery temperature is equal to or greater than a maximum acceptable temperature (T.sub.max) or when the battery temperature is equal to or less than a minimum acceptable temperature (T.sub.min) and, at the same time, when the difference between a battery temperature (T.sub.bat) and an air temperature (T.sub.air) of air is equal to or greater than a critical temperature (T.sub.crt), and operating the fan to change the battery temperature to an optimal range of temperature variation (T.sub.opt1) of each unit cell when the battery temperature does not satisfy the condition, and a temperature variation (T.sub.var) of each unit cell is equal to or greater than a predetermined critical temperature (T.sub.crt1). According to the present invention, the operation of the fan is minimized, and therefore, optimal battery operation is accomplished while unnecessary power consumption is reduced. Furthermore, the temperature deviation between the unit cells is reduced, and therefore, the operation of the overall battery system is optimized.