Construction industry has complexity in its nature because it contains large number of parties as clients, contractors, consultants, stakeholders, shareholders, regulators and others. Construction projects in Iraq suffer from many problems and complex issues in performance such as time, cost, quality and safety. The aim of this thesis is to identify and evaluate the main factors affecting the performance of construction projects in Iraq and comparing these factors with those in the United Kingdom. Literature review about performance was carried out to identify the factors affecting the performance of construction projects. In addition, other local factors have been added as recommended by local experts and according to the researcher's own experience in implementing construction projects. Pilot study of the questionnaire was achieved by a scouting sample, which consist of 9 questionnaire sets in additional to few academic staff in both Iraq and United Kingdom. A questionnaire survey was conducted and 64 factors were identified, categorized into 8 groups, evaluated and ranked from clients, consultants, and contractor's perspectives in both Iraq and United Kingdom. In Iraq 116 questionnaire sets were collected representing 36 clients, 38 consultants and 42 contractors in different parts of Iraq. In the United Kingdom 152 questionnaire sets were received representing 50 clients, 54 consultants and 48 contractors. The practices concerning with the Key Performance Indicators (KPI's) such as time, cost, and client's satisfaction and safety checklists were analyzed in order to know the main practical problems of projects performance in both Iraq and United Kingdom and then to formulate recommendations to improve performance of construction projects in Iraq. It was concluded that projects were delayed and the actual cost of projects was much more than their values because of Iraq's political and security conditions. Overall project safety factors had been moderately implemented in construction organizations. It is recommended for construction organizations to have a clear mission and vision to formulate, implement and evaluate their performance. A structured methodology and technique should be identified to overcome the effect of local political and economic situations on the performance of construction projects in Iraq. In addition, it is recommended to develop human resources in the construction industry through proper and continuous training programs about construction projects performance. It is necessary for construction organizations in Iraq to evaluate both of market share and liquidity before implementation of any construction project because of difficult economic situation. All of that will assist organizations to perform projects successfully and strongly.