Operation of closed-ring power distribution systems has various advantages over both meshed-operated and radially-operated systems. Closed-ring system unlike to radial system, the voltage drop is less, achieves high reliability of power on demand since the power is supplied from both ends, and reduces the voltage fluctuation in high loaded areas by using a tie power line. However, the reliability assessment of closed-ring power grids is not a trivial task. Monte Carlo Simulation (MCS) is one of the most famous methods to assess the availability of any power system. However, the most proposed methods are able to evaluate the reliability of radial and open-ring grids. This paper developed the best known MCS method to assess the reliability of closed-ring grids by integrating the total loss of continuity (TLOC) definition into the MCS. The developed method is called modified Monte Carlo Simulation (MMCS) method. The MMCS is tested by using Roy Billinton Test System (RBTS) buses 2 and 4. The obtained results confirm the correctness of the proposed method. Therefore, MMCS method is appropriate to assess the reliability of both simple and complicated closed-ring systems.