Transmission cost allocation among market participants in restructuring of power industry has become an important task in view of bilateral transaction framework. This paper presents a usage based methodology of transmission cost allocation in deregulated power system under open access. A new methodology for allocating transmission cost to wheeling transactions considering Zbus and contractual obligations made among suppliers and buyers is proposed in this paper. The usage coefficients are defined by taking the quantum of bilateral agreements made in the electricity markets so as to obtain fair utilization of transmission network. Accordingly this new approach calculates the utilization of each line by the generators and loads considering their contract obligations in the open access environment. The transmission costs are allocated among the transactions by using the converged load flow solution in this methodology. In this paper the allocation of the transmission costs mainly emphasize both technical and measure of ‘extent use’ of transmission resources. The proposed method has been illustrated with the help of a sample 6 bus (test) system, IEEE 118 bus system and comparison is done with analytical method for six bus system.