High rate of electricity demand requires stable and continuous supply of electrical power to consumers. Since electric power supply in Nigeria has been erratic and unreliable; this work thereby shows the reliability evaluation of selected hydroelectric power stations in the country, with a view to improve the generation and other system performance by applying probability theories, using Frequency and Duration approach and statistical analysis. The result obtained gives an overview of the general reliability performance of the stations considered; this will thus assist the energy planners and operators in generating capacity planning as criteria for future designs and operation of hydroelectric power stations.