The growing development of gas-fired power plants over the last few decades has enhanced the interaction between power and natural gas sectors. As a result, considering an integrated gas and power network is essential to obtain a more precise and reliable analysis of generation and transmission capacity in the power network. This paper investigates a power generation and transmission expansion planning problem in an integrated and restructured electricity and gas market. To model interactions between the different participants of the markets and to identify the equilibrium point, a mixed complementarity problem approach is applied. This paper exploits Cournot game models to provide a behavioral framework for modeling of imperfect competition among electricity producers. Iran's real natural gas and power systems are used to test the proposed model, and its validation is conducted by comparing the pattern of the model outcomes with Iran's real data. The results indicate that expansion decisions lead to lower electricity price as well as improvement in social indexes such as consumer benefit and social welfare. In addition, it is revealed from the results that higher gas price due to low production capacity can lead to a change in the equilibrium point of the power market. (C) 2018 Elsevier Ltd. All rights reserved.