A key concept in restructuring of power industry is Available Transfer Capability (ATC). In several decision making problems and from different viewpoints, the ATC plays an important role and should be computed. In this paper the ATC is determined and analyzed from the planning aspect. The problem is formulated for a regional power system, namely Khorasan province transmission network. Computing of ATC through a specified path which determined in the network formulated as an optimization problem. The aim is to maximize amount of transferable power through the specified path subject to the load flow equations and transmission constraints. Regarding to geographical position of the province, the model is generalized to consider simultaneous transfer capability. Using a probabilistic model, effects of single contingencies on the ATC will be discussed. The Genetic Algorithm as a powerful optimizer is used for solving the problem. Analysis of results shows the proposed method is practical and it would help system planner in the expansion planning process.