Over the recent years, the cost escalation in oil pipeline projects in Iran has been inevitable due to the political and economic conditions. On the other hand, due to Iran's 20-year vision plan, several projects and plans have been defined by National Iranian Oil Company (NIOC); therefore, to achieve the vision plan's goal and deliver the projects on-budget, identification of the cost overrun causes is the first step. In this research, a list of important cost overrun factors in oil pipeline projects was extracted after a detailed literature review, study the executed projects document, semistructured interview, and a questionnaire survey among clients, consultants, and contractors who are involved in such projects. The results show that the political and economic sanctions, rise in the price of material, delay in payments, unrealistic price proposal to win the tender, inflation, change in orders/extra works, type of bidding award, delay by vendors/suppliers, slow speed in obtaining permits/massive bureaucracy, and unrealistic time and cost estimation are ranked in the top ten cost overrun factors.