A good level of reliability will determine the continuity of the electrical distribution of the system. With the availability of power that is not sufficient then it will affect the level of a reliability system. One method for calculating reliability is the Loss of Load Probability method. With a low risk of interference will be obtained a high level of reliability in the operation of electric power systems. In Steam Power Plant PT. PJB Generation Unit Gresik obtained LOLP value of 31.5 days / year in 2017. LOLP index value has not met the standards set by PT. PLN in RUPTL PLN 2015-2024 is 1 day / year, based on it can be said reliability of power system at PLTU PT. UP PJB Gresik in 2017 in the category of less reliable or low power plant reliability.