This paper presents a computer model for assessing the reliability of uninterruptible power supply (UPS) system in developing countries using the part stress method. Data on failure of APC BK650MI UPS in Nigeria were used as a case study. Special consideration was given to factors like environment, quality of utility and service personnel. From the point of view of part stress, the block diagram of a UPS comprise basically of two sub-units: the charger and the inverter, consequently, a mathematical model used to determine the reliability of the UPS, can by used for another UPS as long as the block diagrams are identical, the number and the quality of constituent parts are taken into consideration in each case.