Semantic; Trust Management; RBAC; User-role Assignment Abstract. The application of RBAC in access management of the enterprise services and resources is very widely. With phenomenal growth of information interaction and cooperation between distributed systems, the number of users can be in the hundreds of thousands or millions. This renders manual user-to-role assignment a formidable task. In this paper, we propose a semantic and trust based framework to automatically assign users to roles based on a finite set of assignment rules defined by authorized people in the enterprise. These rules take into consideration the attributes users own and any constraints set forth by the enterprise including the assignment history and the credit of the requester. We choose OWL to specify the user attributes.