This study investigated the self-understanding and communication style of sixth-grade students, focusing on three points. First, we used egograms to understand students’ personalities and promote their self-understanding. Second, we measured students’ assertiveness and investigated the relationship between assertive behavior and students’ egogram patterns. Third, changes in assertive behavior by intervention intended to promote reflection on one’s communication style were examined. The egogram is a potentially effective tool for increasing students’ self-understanding because the students were highly interested in egograms. Assertive students tend to score highly in the NP of egograms. Non-assertive students tend to get high scores in A. Especially aggressive students tend to get high scores in CP and A. Our intervention decreased the aggressiveness of the students who chose an aggressive style of communication.