The construction industry is a major part of the Jordanian economy. Contracting companies carry out important construction projects in cooperation with subcontractors, that makes subcontractor selection is an important part of the project management. Consequently when awarding construction contracts depending on the price only, it is not always a successful strategy and it may result in many problems like: delay, cost overrun, and bad quality. There are many aspect beside price should be taken into consideration when selecting the suitable subcontractor like: quality, time, and safety. The main objectives of this research are to identify and rank the factors used by main contractors in the selection of suitable subcontractors in Jordan, and to propose recommendations for improving the selection of subcontractors. From previous literature the study revealed the important factors used by consultants and main contractors for selection of prequalified subcontractors. A structured questionnaire was constructed to rank the importance of each of these factors in Jordan, and then a model using linear programming (Assignment Model) was constructed to study many cases toward subcontractors' selection. Also the effect of selecting subcontractor based on prequalification criteria and optimization procedure was studied. The study found the important factors used by main contractors for selection of suitable subcontractors include: "estimated time of project, estimated tender price, technical ability, financial ability, management capabilities and safety", respectively. The evaluation technique used in this research combined the price of bids and the time needed to finish the project beside the prequalification technique; others research don't combine these in one model; all of them starts with prequalification technique for the financial and technical ability then the lowest bid price is selected. The study is also found that distribution of bids among subcontractors is different when the prequalification is used and the total cost of all projects will differ too. The most important recommendation to Contractors is to select the subcontractor according to his previous experience, reputation and capabilities in terms of labors, equipment, and machinery, since these items ensure completing the works according to the time schedule and achieve the best quality. On the other hand subcontractors are recommended to use qualified labors, and good materials to ensure satisfying the required quality, in order to be able to adhere to main contractor requirements and time schedule. Key word: subcontractor, prequalification, Optimization, selection criteria, Linear Programming, Assignment Model.