The paper presents reliability study of Uninterruptible Power Supply (UPS) system configurations. The five main UPS system design configurations namely Capacity, Isolated Redundant, Parallel Redundant, Distributed Redundant, and System plus System Redundant were considered and comparisons on the resultant system's reliability parameters were discussed in detail. The results obtained from this study can play an essential role in the selection and application of the UPS systems. UPS reliability parameters such as failure rates, Mean Time Between Failures, and Availability can be estimated from the proposed system's reliability estimation methods. The suggested reliability estimation methods in this paper are the Reliability Block Diagram (RBD) and the Monte-Carlo Simulation methods.