Frequency analysis of city gas pipeline was studied and then the method to give frequencies of failure by the third-party digging, corrosion, ground movement, and equipment failure which were known to be the major cause of risk of city gas pipeline. The failure by the third-party digging was analyzed by fault tree analysis and the failure by corrosion was analyzed by applying equation calculating remaining strength with time. The failure by ground movement was evaluated by applying modified model which was induced through weighing factors with basic failure rate model. The failure rate of equipment was calculated with both generic and specific data