Due to the limitations in fossil fuel resources and growing need to new energy resources, and to reduce greenhouse gasses, using renewable resources in electric power systems has become very attractive in recent years. In this way, the wind power generation has gotten the most attention. However, because of the variation of the amount of produced power due to changes in wind speed, the operation of the power system faces new challenges which make reliability studies more important. In this study the reliability of offshore wind farms is studied and the influence of these farms on system adequacy is estimated and compared with onshore wind farms. For this purpose, we firstly describe the characteristics of offshore wind power plants and then, by using the capacity outage table in a recursive process, we obtain the overall model of offshore wind power plant based on a Markov chain. In this research, to show the impact of offshore wind power on power system adequacy indices, we simulate the proposed method in the RBTS test system. For this purpose, at first the adequacy indices of this system is calculated and then by adding an offshore wind power plant to the typical test system, its effects on reliability indices will be estimated. In modeling offshore plants, the effect of the wind turbine hub height, power collection grid and mean time to repair changes throughout one year and in different weather conditions are considered. The results indicate that higher levels of wind speed in offshore wind turbines, improve the system adequacy more efficiently than onshore wind power plants. This improvement is to such extent that the increased errors caused by changing weather conditions and adding additional equipments such as complicated power collection grids, is neglected. Keywords : power system reliability, offshore wind power, production adequacy