In this paper, three performance indices of a transmission line i.e., line security margin, line security index and line severity index are proposed to be used for transmission system planning. The objective is to create systematic indices to assist the planners to identify vulnerable power flow paths of a system which may lead to voltage instability problem from proliferated demand. The calculation framework begins with identifying loading scenarios on a time horizon. Hence, these performance indices can be calculated and ranked along the loading trajectories due to their severity. The proposed methodology is tested with a 28-bus system simplified from a part of Thailandpsilas system. Identifying for the critical flow paths is analyzed with some recommendations to strengthen the system. Accordingly, transmission expansion or upgrading projects can be instigated. Eventually, the obtained result is compared with the recent Thailand power development plan (PDP) with interesting results.