With the need for improved performance in the construction industry and increasing contractual obligations, the requirement of an effective risk management approach has never been more necessary. In this paper, a new approach to construction project risk assesment based on rough set and information entropy is proposed. Firstly, the construction project risk evaluation index system is established, and then the indexes are reduced with no information loss through rough set approach. Secondly, risk factor weights are qualitatively described with information entropy. Thirdly, the qualitative results are transformed to quantities value and the result of evaluation is worked out by using unascertained number algorithm. Finally, a case study was carried out on the risk assesment of a sample project using the prototype. The results show that the rough set and information entropy can help understand the uncertainties in construction risk assesment, and the relationships between risk sources and the consequences on project performance measures can be identified and quantified consistently. © 2008 IEEE.