A power system must be able to deliver the electric power even though some disturbance occurs. The failure of the protection system to localized fault can cause catastrophic failure of a system or blackout. In this research, an algorithm based on Monte Carlo simulations is proposed to obtain the reliability index that considers the overcurrent relay coordination varied by time settings of overcurrent relay. The reliability index consists of SAIFI, SAIDI, and CAIDI. Monte Carlo is a simulation procedure to obtain the reliability index by using a random number of generation and a simulated through a program. The results from Monte Carlo simulations is to determine the disturbance in a transmission line, the overcurrent relay that works at the time, and the reliability index on consumer that the parameters is derived from the amount of the failure rate and the rate of improvement of the line and transformer. The proposed method is compared to commercial software that is ETAP®. The results of ETAP simulation only count the failures in transformers, and the outcome of SAIFI and SAIDI is 0.013 and 0.1819. Other ways on the Monte Carlo simulation to determine disturbances in line with the generation of random numbers on the 2000 iteration, value of SAIFI and SAIDI is 0.0139 and 0.2697. The strength points of this study are implementing and analyzing the method into real system in Indonesia.