The South Pars Gas reservoir, as the largest natural gas supply source in the country, is one of the most important mega projects in Iran that has been used to develop its phases by various contractual methods. In this research a comparison between methods and the most appropriate contractual method has been identified which could be the basis for future development of gas fields or mega projects in the country. With the inclusion of criteria such as project risk for government, cost, quality, the integrity of implementation and production operations, project execution time, acquisition management know-how, by using “ Analytical Hierarchy Process” (AHP) method for contractual options, shows the superiority of the buy-back contract. The main features of the research that led to this conclusion are, the maintenance of the country's right to ownership, control of operations and costs by the state, transfer of technical know-how, acceptance of the risk of production and Perform 100% investment by the contractor and repayment it within 5 to 7 years after the commissioning of the field products.