Construction projects have become more complex, and worries about the conception of project complexity have increased, thus project complexity has become a crucial factor in management of project that poses extra challenges to achieve project goals. Understanding complexity is of great importance to practitioners in the field of project management because of the differences related to making decisions and achieving goals that seem to relate to complex projects. Complexity affects project planning and control. It can influence the choice of an appropriate model for project organization, or it can even impact project results. Complexity can also affect project modeling, evaluation, and control, and time, cost, quality, and safety objectives. It is necessary to define the concept of complexity for construction projects, identify the factors that affect complexity. A number of methods have been used throughout the research process, interviews and questionnaires used to determine the project's complexity factors. The objectives of the research include the following: (1) Defining the complexity and characteristics of the project; (2) Determining the importance of the complexity factors which are affected by the complexity of the project and (3) ranking the complexity factors according to the degree of their influence on complexity. Hence the results of the research can help scholars and practitioners in the field of project management to understand the crucial factors of project complexity and develop an appropriate strategy for effectively managing the projects complexity.