Transmission network expansion planning (TNEP) is a basic part of power system planning. It determines where, when and how many new transmission lines should be added to the network. Different approaches have been proposed for solution of the static transmission network expansion planning (STNEP) problem up till now. However, in all of them, the effect of bundle lines in transmission expansion planning considering the network losses has not been studied. In this paper, the effect of bundle lines on STNEP has been investigated considering the network losses by decimal codification based genetic algorithm (DCGA). The proposed idea is tested on an actual transmission network of the Azerbaijan regional electric company, Iran. The results evaluation show that bundle lines considering the network losses have important effects on TNEP problem. In addition, considering the bundle lines in a multiple voltage level transmission network with different annual losses is caused that the total expansion cost of the network (expansion and operational costs) is decreased and the lines is overloaded later and therefore network satisfies the requirement of delivering electric power more safely and reliably to load centers. In addition, the network adequacy increased and therefore the transmission expansion planning is optimized. (C) 2010 Elsevier Ltd. All rights reserved.