Access control in Multi-Organization Environment is a critical issue. Classical access control models like Role Based Access Control (RBAC) and Organization Based Access Control (ORBAC) need some improvements to be used in such environment, where the collaboration is established between organizations and not directly with the clients. In particular, some characteristics of this scenario are that the users may be unknown in advance and/or the behaviors of the users and the organization may change during the collaboration. Hence, in this context the use of trust management with an access control model is recommended. To achieve this goal in this paper a new model called Trust-Orbac that adds the notion of trust management to ORBAC is presented. This approach consists in defining two dynamic trust vectors: one for the organizations and one for users which are based on different parameters such as knowledge, reputation and experience. Finally, we illustrate the use of Trust-Orbac with a case study.