This paper describes a methodology for designing a cost-efficient transmission system configuration on a green field where we consider land condition, generator condition, load condition, and acceptable reliability. The reliability constraints include static constraints under normal and N-1 conditions, and transient stability constraint for three-line-to-ground faults. Under the N-1 condition, power generation rescheduling is considered to obtain a solution at lower cost. The proposed method divides the design problem into two stages. The size of the problem is reduced by applying Dijkstra's algorithm in a preparatory stage, and then, the problem is solved by a PSO-based algorithm in an optimization stage because we have to deal with integer variables and differential equations. The proposed algorithm is tested for a small green field with three generators and three loads, and the results show impacts of the generation rescheduling and the transient stability constraint on the designed system configuration. Finally, we verify the effectiveness of the proposed method for a larger green field modeled based on north-east Japan system, and show that the algorithm can solve the problem with a reasonable cost-effective solution.