This paper proposes an optimal transmission expansion planning (TEP) which is based on determining share of each line in merchandizing surplus (MS) of system, determining by Independent System Operator (ISO). The more share of a line in MS of system denotes the priority of a line for expansion. The procedure of determining MS of each line in a power system is based on determining the MS share of each energy exchange between certain generator and certain demand bus in the power system. By analyzing all energy exchange the optimal planning of transmission line is obtained by ISO. The variable revenue of a Transco is related to performing the optimal planning of transmission lines which is obtained through ISO. The proposed method determines the TEP economical resources and the procedure of receiving these resources (MS of system) from generators and customers. By performing proposed method Transco capacity withholding and misusing is prevented spontaneously caused by relating the revenue of Transco to its optimal performance. Manuscript profile