According to researches, oil and gas projects generally and upstream projects of them especially include a lot of complexities and uncertainties, which lead to an increase in the risk of these field investments. Despite, using risk management methods and techniques, with applying hardware and software developments, has become more popular, recent researches have not been able to provide the comprehensive view of oil and gas upstream risks. As respects to the important role of these projects for Iran economy and necessity of mass investments in the oil and gas upstream sector, it is necessary to identify, assess and prioritize the oil and gas upstream risks methodically and structurally. In this study, with using risk break down structure framework based on Project Management Body of Knowledge (PMBOK) and PEST Classification, the oil and gas upstream risks would identify and classify with a documentary method. The study identifies 60 risks and classifies them into four classes, which have been used in next step for prioritizing by TOPSIS technique. The results with prioritizing based on third risk breakdown structure level, show that as well as the diversity of and variety of upstream project’s risks, there are the heterogeneous risks from first and the second risk breakdown structure level. These results emphasize on selecting precise and appropriate tools to the management of risks