Transmission network expansion planning (TNEP) is an important component of power system planning. It determines the characteristic and performance of the future electric power network and influences the power system operation directly. Up till now, various methods have been presented for the solution of static TNEP (STNEP) problem. However, in all of them, the role of bundle lines in TNEP problem considering the expansion of substations from the voltage level point of view has not been investigated. Thus, in this paper, the role of bundle lines in STNEP problem is being studied considering expansion cost of substations from the voltage level point of view using decimal codification genetic algorithm (DCGA). The effectiveness of the proposed idea is tested on an actual transmission network of the Azerbaijan regional electric company, Iran. The results reveal that bundle lines have effective role in transmission expansion planning and subsequent determining the network arrangement. In addition, considering the bundle lines in a power system is caused that the expansion cost of lines and substations decreases and therefore the total expansion cost of network is minimized. Also, it can be said, although cost of bundle lines are more than those which have not bundle conductor, constructing this type of lines in a transmission network with different voltage levels prevents useless expansion of unbundled lines in separate corridors and therefore the network expansion planning is optimized. (C) 2009 Elsevier Ltd. All rights reserved.