This paper proposes a new methodology for evaluating the probabilistic reliability of a grid constrained composite power system including wind turbine generators(WTG). The proposed model can consider capacity limitations and transmission line forced outage rates. The importance of renewable energy sources has been growing at a high rate as a result of being environment friendly. In particular, wind energy is one of the most successfully utilized such source to produce electrical energy. The random nature of wind speed can be incorporated in the evaluation by representing the WTG by multi state models. A simple case study shows in detail how it is used to perform a reliability evaluation study in proposed model.