Cost overrun in construction projects is a common phenomenon in Iraq. This might occur due to diversity of factors. This study aims to identify the factors influencing construction projects cost that are potentially controllable by main contractors. A field study through a questionnaire survey was directed to a sample of related Iraqi professional engineers from general contracting companies at both public and private sectors. Their opinions on the impact and frequency of each factor were investigated. The questionnaire offered (59) factors classified in (8) categories namely; legislations, financial and economic, design, contractual, site management, material, labor and equipment. The factors were ranked according to the highest Relative Importance Index (RII). The study revealed (10) major factors that are potentially controllable by main contractors namely; labor productivity, sub-contractors and suppliers performance, equipment productivity, site organization and distribution of equipment, experience and training of project managers, scheduling and control techniques, planning for materials supply, planning for equipment supply, materials delivery and planning for skilled labor recruitment. Recommendations to aid contractors and owners in early identification of these factors are also included in this study.