The delays in construction projects affect a very large volume of the countries’ resources, therefore making the right decisions at managerial levels prevents the loss of national resources. There are several tools available to project managers for appropriate resource planning, each of which is used at managerial levels to improve the schedule of projects. In fact, delays can be largely prevented if there is an appropriate decision-making model in place to control the projects. This study is aimed at investigating the causes of delays in construction projects through the case study of the National Iranian Oil Company's construction projects by using a hierarchical analysis method and prioritizing the most important delay factors as well as providing a solution. After the background studies, the raw data of the research was collected using 64 questionnaires. This includes valuable experiences of experienced administrators and executives. Then, the preliminary data were analyzed by Expert Choice software, and a model was presented for optimizing the problem in order to prioritize and determine the importance of the causes of project delays. Given the software output, the results showed that the factors affecting the increase of the duration and cost of the projects include the following: 1) Existence of sanctions and restrictions on imports of essential goods and commodities, 2) Governmental policymaking problems in prioritizing the construction projects, 3) Social and environmental effects and conflicts, 4) Financial insolvency of the contractor, 5) Lack of financial resources of the employer, 6) Inconsistencies between consultant and employer, 7) Study and design problems, 8) Lack of specialist and skilled manpower by the contractor, 9) Higher workshop supervision problems, 10) Contractor’s technical and equipment problems, 11) The type of contract and contractual issues, and 12) Managerial and technical inability of the employer, the impact of which are 1) 17.3%, 2) 13%, 3) 9.6%, 4) 8.9%, 5) 8.3%, 6) 8.3%, 7) 7.3%, 8) 6.6%, 9) 6.3%, 10) 5.6%, 11) 5.1% and 12) 3.7%, respectively. The results of the study suggests that the sanctions and restrictions on the import of essential goods and commodities, and problems of governmental policymaking in prioritizing the construction projects are superior to other factors in most cases in terms of the studied criteria (time and cost), as well as the case study. Also, other specific project conditions can change these priorities. As a result, project managers should consider the analyses carried out in this research with specific conditions of each project simultaneously and make decisions.