By virtue of the vital nature of electric power, both to the economic and personal wellbeing, a power system is expected to supply electrical energy as economical as possible and with a high degree of quality and reliability. But electric supply in Ethiopia has been erratic and unreliable due to faults inherent in the long operation of the station and aging of their associated auxiliary equipment often lead to forced outage of the generating unit. This thesis presents reliability evaluation based on probabilistic and analytical method used to assess and evaluate the reliability, availability and performance index parameters. The research aimed at evaluating the reliability performance of Beles hydroelectric power station of Ethiopia. A set of reliability parameters which quantify generating unit reliability were computed for each generating unit using the annual outage duration of Beles hydro power stations for period of study 2014 - 2017.The most important reliability indices like, failure rate (λ), repair rate (μ), mean time to repair (MTTR), mean time between failures (MTBF), mean time to failures (MTTF) have been determined through data collection and analysis. The data of each year for each unit is time scheduled. The results showed the average reliability of the four units of the plant as 0.872, 0.860, 0.899 and 0.886 for units 1, 2, 3 and 4 respectively while the overall reliability was 0.879 and the plants average availability were 0.8476 for the period of study .The generation loss analysis indicated that inadequate water in the reservoir, lack of spare part, grid constraints and plant unavailability prevented the plant from running at maximum continuous rating.