This work is a reliability assessment of Nigeria's existing power system. A probabilistic framework involving Markov reliability models is used to evaluate the ability of available generation to serve Nigeria's existing power demand. Resource planning indices, such as reserve margin and loss of load probability (LOLP), were evaluated. This study also investigates the sensitivity of LOLP to three base load levels (2900, 3100, 3225 MW) and unit availability assumptions (65%, 80%, and 95%). Results show that loss of load probability is extremely sensitive to generation availability. For the 2900 MW base load case, LOLP is reduced from 83.26% to 20.50% moving from 65% to 95% unit availability. Conclusively, in efforts to improve Nigeria's overall system reliability, it behooves resource planners to focus on improving the unit availability levels of existing generation even as new reliable power resources are installed. Such an energy development strategy is likely to be more effective at improving Nigeria's ability to serve its ever-increasing load demand. This work can be improved by considering transmission line effects and evaluating the economic value of reliability in Nigeria.