A methodology to quantify the economic risk of a natural gas transmission pipeline system is developed in this study, and the effect of the line pack is considered. The methodology is based on the risk theory and hydraulic analysis of the transmission pipeline system. In the methodology, the failure probability of the key components in the transmission pipeline system are estimated firstly. Moreover, the transient compressible model for the pipeline system is employed to perform the consequence analysis, and the model is solved by the Semi-Implicit Method for Pressure-Linked Equation (SIMPLE) algorithm in the process of the consequence analysis. By combining the failure probability estimation with the consequence analysis, the economic risk when an accident happens can be evaluated. A detailed procedure for economic risk evaluation of a natural gas transmission pipeline system is presented, and its feasibility is confirmed with a real transmission pipeline system. Furthermore, impacts of the line pack on the economic risk are investigated, and suggestions to decrease the economy risk are proposed. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.