Transmission and distribution systems are linked to each other through the sub-transmission network, where it receives the electric energy from the transmission network at extra high-voltage levels, and delivers it to the distribution network at low-voltage level. Hence, if the sub-transmission network is designed in an optimal manner, it will supply the loads of distribution system adequately on one hand, and this leads to an efficient design of transmission network on the other hand. As the design optimality of transmission, sub-transmission, and distribution systems are dependent on each other, the best result for the network expansion is achieved when these three networks are considered coordinately. Since implementing the coordinated design of transmission, sub-transmission, and distribution systems is very complicated, traditionally, they are designed separately. In this paper, a new formulation has been developed for the coordinated expansion planning of transmission substations, sub-transmission network, and distribution system. In addition, to obtain more efficient results for the network expansion, distributed generation (DG) units are considered in the problem. The proposed approach is implemented on a real network of Zanjan Regional Electrical Company, and the results are discussed. The obtained results demonstrate the effectiveness of the conducted planning method. (C) 2016 Elsevier Ltd. All rights reserved.