With the rapid development and the extensive application of vehicular ad hoc networks(VANET), the problem of  how to ensure the credibility of the vehicular endpoints during access authentication has become the research focus. Because the rough set theory is suitable for handling uncertain information, we propose a new approach to accessing VANETs based on the rough set theory. The mobile trusted module(MTM) on endpoints in VANETs is used for collecting the trusted attribute information, and the final accessing decisions can be made. The experimental results show the  effectiveness and practicality of this method.