Abstarct: Nowadays, we have witnessed an increase in the usage rate of wind energy, mainly due to the increasing need of human to energy along with economic and environmental considerations, and has drawn a lot of attention throughout the world. We, therefore, due to the wind farm extension requests and an estimated increase in Khaf's electrical energy demand, here in this thesis, study the extension of Khorasan Electrical transmission system baxsed on system reliability. First to get a sense of the existing possibilities and capacities created from the Electrical Policy Making Company, we study the possibilities and potentials of the area's wind-energy capacities baxsed on the updated data. It is done in a frxamework with recognition of uncertain factors respect to regional lands situation and calculation of the maximum extractable wind power in the Khaf region. Forasmuch as the large scale presence of wind power in this study, the correlation between wind power and electric load has been checked for the specific buses in Khaf area. In this regard, existing uncertainties in the investment policies of the government and private sectors makes planning’s time horizon impossible to determine. To cover and compensate for these uncertainties, various scenarios at different levels of wind farms and grid electric load growth has been developed. Moreover, in order to reduce the uncertainties, a model for wind farms is proposed. Eventually, the reliability of the Khorasan transmission is scrutinized with special attention to distribution network in the Khaf region in the presence of wind farms. Distribution lines loadings, network configuration and wind farms and their impact on the reliability index have been evaluated.