Recently, the highway construction projects in Iran have been confronted with delay, terminated, or postponed for several years. Although widespread studies have been investigated, regarding to delay issue in developed and developing countries, however, only few studies addressed in Iran. Due to the importance of delay issue and lack of sufficient studies regarding to this topic in Iran, this study aimed to first, discover the most significant causes of delay in highway construction projects, second; prioritize, and rank the critical factors, in order to highlight them for highway construction projects authorities. A verified questionnaire prepared and distributed among the experts in the construction projects of Iran. The valid collected data analyzed by Exploratory Factor Analysis (EFA) to uncover the underlying structure of variables and reduce the number of them. Moreover, Analytical Hierarchy Process (AHP), as one of the prevalent decisions making tools, applied to prioritize and rank the significant factors. The study results revealed that the "Design" and "Contractor" were manifested as the most critical criteria regarding to delay in highway construction projects in Iran. Besides, poor procurement of construction materials, delay in manufacturing materials and accidents during construction were discovered as the most significant factors causing delay. This study contributed first to identify and prioritize the most significant causes of delay in construction projects in Iran, as a developing country, because of lack of sufficient studies regarding to this topic. Additionally it highlights the causes of delay for any involving parties in Iran construction projects in order to eliminate delay.