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Transmission open access is one of the essential issues in restructuring of electric industry. In this way, pricing of transmission system should be performed in a fair and reasonable procedure. This paper describes the principle and implementation of a reliability-based methodology for allocating the cost of transmission network. A probabilistic index is defined which measures the relative importance or reliability benefit of the unused (additional) capacity of each particular line for each transaction. The index is calculated based on contingency conditions analysis corresponding to line outages. Transmission embedded costs are allocated based on the contribution of each transaction to base capacity in normal condition and also to the additional capacity in contingency conditions using the defined index. Furthermore, the split between the capacity use and reliability margin costs is performed on a reasonable decision. As a result, the proposed methodology provides an equitable means of allocating the cost of transmission network among its users. The methodology is illustrated in a small power system and its conceptual and computational feasibility are shown.