Abstract: The large-scale utilization of natural gas units continuously increases the interactions between power systems and gas systems. Considering that, random failures may propagate between energy subsystems, bringing new requirements for long-term planning of the electricity-gas systems (EGs). In order to improve the reliability of EGs, this paper proposed an energy capacity planning model considering multi-objective coordination (i.e., reliability and costs) to determine the location and capacity of generators and energy storages. Considering the failure characteristics of components, multi-state reliability models were proposed to describe the transition process among different operating states. Based on the reliability models of components and multi-state energy loads, the reliability constraints of gas system and power system were sequentially formulated, where the cross-sectorial failure propagation was considered. Besides, the energy capacity planning model of the EGs was proposed to minimize the total system costs, including investment costs, operation costs and load curtailment costs. In the proposed model, the investment constraints, operation constraints and reliability constraints of IGPSs were considered. Finally, a case study composed of IEEE 30-bus system and a 20-bus Belgian gas network was utilized to validate the effectiveness of the proposed model.