Abstract: With the intensified integration of electricity and natural gas system, any disturbance or failure of the vulnerable line in one subsystem may propagate to another system, consequently resulting in a failure of the whole integrated system. To this end, a novel method to identify vulnerable lines of integrated electricity and natural gas system was proposed. Firstly, a stochastic co-optimization model of integrated electricity and natural gas systems considering wind and photovoltaic generation uncertainties was constructed to access the normal and contingency operation conditions of the integrated system. Specially, vulnerability of lines in the integrated electricity and natural gas system was discussed from the point of topological vulnerabilities and functional vulnerabilities via local centrality and power flow changing entropy, respectively. The line vulnerability index, considering both topological and functional vulnerabilities, was constructed to identify vulnerable lines for the integrated electricity and natural gas system. Finally, the geodesic vulnerability and generation/load imbalances were used to quantify influences when the vulnerable lines are out of service. Numerical results of a 6-bus electricity/7-node gas system and a 24-bus electricity/20-node gas system illustrate the effectiveness of the proposed method.