Facing the unceasing challenges of new technology, the use of virtual teams as the future organization structure has become more and more common. The objectives of this thesis are to investigate members’ behavior in each stage of group development adopted from Tuckman’s five-stages model, and to find out the impact of critical behaviors on virtual team’s collaborative performance. This thesis selects nine learning-task oriented virtual teams composed by part-time graduate students in cyber university of NSYSU and probes into the theme of each groups’ discussions by content analysis approach. SYMLOG is applied to observe group member’s behavior and construct the categories of content analysis. According to the research results, the critical behaviors of initial stage are active (U) and friendly (P) behaviors. During the middle stage, the existence of encouragers and followers should improve group development towards high performance. In the final stage, group members shall be able to shift the communication focus from emotionally expressive (B) to instrumentally controlled (F) in order to reach superior effectiveness. Therefore, the findings of this thesis will provide some suggestions about member’s behavior, facilitate group development, and improve virtual team’s collaborative performance.