Project risk is an uncertain situation or event that, if it occurs, may have a negative or positive effect on one or more project objectives, such as scope, schedule, cost, and quality. Major industrial projects are increasingly facing complexity and uncertainty. The scope of this paper is related to petrochemical projects, in which risks directly affect the approved time, cost, and quality of the project. In such projects, there are risks that neither the owner nor the contractor has the main role in the occurrence or prevention of, and it is not easy to determine who is responsible for them. In such projects, there are risks that neither the owner nor the contractor has the main role in the occurrence or prevention of, and for which it is not easy to determine responsibility. Therefore, predicting, identifying, analyzing, and determining of the optimal allocation of risk responsibility between contracting parties is one of the most important steps before the start of the project. Suppose it is not correctly allocated among project stakeholders, then, in that case, risk responsibility imposes costs on the project that must be paid by the owner, contractor, and partnership, causing, in general, many problems for project management. Therefore, this paper presents a model to calculate the optimal ratio of risk allocation between the project parties in the concluding contract stage, using the UTA-STAR technique to obtain the owner and contractor utility function to create as much of a win-win relationship between them as possible.