The size of the power system is growing exponentially due to heavy demand of power in all the sectors (e.g., agricultural, industrial, and commercial). Due to this, the chance of failure of individual units leading to practical or complete collapse of power supply is common to be encountered. The reliability of power system is therefore the most important feature to be maintained above some acceptable threshold value. Furthermore, the maintenance of individual units can also be planned and implemented once the level of reliability for given instant of time is known. The proposed research therefore aims at determining the threshold reliability of generation system. The generation system consists of boiler, water, blade angle in turbine, shaft coupling, excitation system, generator winding, circuit breaker, and relay. This chapter presents the mathematical model of reliability of individual components and equivalent reliability of the entire generation system. It suggests the approach to determine the critical reliability of both individual and equivalent reliability of the generation system.