As social networking services are popular, the need for recognizing reliable people has become a main concern. So, trust plays an important role in social networks in order to recognizing trustworthy people. The purpose of this paper is to propose an approach to recognize trustworthy users and protect users from misused by untrustworthy users. In this paper, we suggest a method to measure trust value based on call log histories and QoS requirements. After that, we calculate error-hit, precision and recall. Also, the trust issue in social networks is pointed out and the new approach to evaluate the trust based on call log histories and QoS requirements is proposed. The results indicate that the proposed approach has better error-hit, precision and recall than the other four models (FIFO, combined, QoS-based and call log-based).