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The purpose of this paper is to describe and discuss an approach to estimate the Reliability and Availability values of the Uninterruptible Power Supply (UPS) systems using Monte-Carlo simulation method. In this method, the failure rates (λ), and availability (A) of the major components in the UPS are simulated by the use of random number generators that produces deterministic reliability parameters that try to mimic results that would be obtained from field trials. Reliability parameters such as the system's Failure Rates (λ), Mean Time Between Failures (MTBF), System Availability (A), and System Unavailability (W), can be achieved and thus can be used as an indication of the reliability level of the UPS systems. In this paper, the method are applied to five UPS configurations (online with and without bypass, offline, line-interactive with buck/boost transformer and line-interactive with ferroresonant transformer), and comparisons on the results obtained from this method were made to the results from other reliability estimation method such as the Reliability Block Diagram (RBD), Boolean Truth Table and Field Data estimation method.