In a competitive power market, it is important to expand or strengthen transmission system in order to provide a fair environment to all market participants. In the present paper, a new method to determine an optimal transmission expansion plan in the deregulated power system has been proposed. In the proposed scheme, uncertainties, security index and transmission costs in calculation of LMPs for transmission expansion planning are taken into account. Considering probability density function, the objective function has been formed and can help to calculate LMPs. For minimizing objective function harmony search algorithm that has better convergence in proportion to other algorithms, has been applied. Simulation results on IEEE 30 bus network have been performed. After calculating of LMPs, candidate lines with minimum MLMP (mean of LMP) have been selected.