The Influence of Contractors Involvement in Engineering Procurement Phase to Construction Project Cost Performance in IndonesiaThe structure of the construction industry is very complex due to the involvement of a wide range of contractors including main contractors, subcontractors and design consultant. The amount of participants in the actual construction process requires various relationships and joint combination often leading to lower levels of performance in construction projects. In Indonesia, there are many construction projects that are performed in a traditional manner, where designers and contractors are given work in separate contracts. Contractors generally get involved in the construction phase. After completing planning, design and specifications, the designer embarks on supervision in the construction phase. In 1986, The Construction Industry Institute, in Austin, USA, defined Constructability as the optimum use of construction knowledge and experience in planning, engineering, and procurement and construction field operations to achieve overall project objectives. This research was carried out in order to complete the requirement of Magister Program Curriculum in The Department of Civil Engineering, Postgraduate Program of The University of Indonesia. It is mainly aimed to investigate the relationship between the contractors personnel involvement in engineering and procurement phases with the construction project cost performance in Indonesia. Data of the research were obtained through questionnaires returned from contractor companies that performed many kinds of construction projects completed over the last decade between 1987-1997 in Indonesia. The method chosen for the research was a descriptive-analysis type relying on data compiled and verified from the samples of the construction projects. Both quantitative and qualitative analysis utilizing correlation and multiple regression to establish a model of the relationships. The final model after being subjected to the needed statistical tests provided clear evidence about the influence of constructability variables as in contractor personnel involvement in engineering and procurement phases to construction project cost performance in Indonesia The significant contractor personnel variables were found to be carefully assign appropriate construction personnel who has the required experience and team approach to the project team and make timely input to design to avoid the need for changes.