Purpose: The purpose of this study is to investigate the causal relationship between the fun, friendship, and pro-social behavior of middle school students participating in free-grade sports activities.BR Method: 397 samples of middle school students who participated in free-grade arts and sports activities were gathered. After completing the questionnaires using the self-administered questionnaire method, 337 valid samples were obtained, excluding 60 questionnaires judged to the inappropriate due to missing or biased answers. The data was processed using the program “SSPS 23.0” to conduct a questing factorial analysis and credibility analysis. The significance level was set at α=.05. In addition, Varimax, an orthogonal rotation method, was performed to confirm the single dimensionality of each component, and the Cronbach’s α value was checked to verify the reliability among variables.BR Results: The results of this study are as follows. 1) Interpersonal relationships, which are sub-factors of fun, were found to have a significant effect on all sub-factors, such as friendliness, self-exposure, and competence. Daily escape was found to have a significant effect on friendliness. 2) Interpersonal relationships, which are sub-factors of fun, were found to have a significant effect on all sub-factors, such as helping, sharing, comfort, and cooperation. Daily escape was found to have a significant effect on helping, physical strength/health, and winning/competition. 3) Competence, a sub-factor of friendship, was found to have a significant effect on all sub-factors, such as giving help, sharing, comfort, and cooperation. Self-exposure was found to have a significant effect on giving help and cooperation.BR Conclusion: In summary, the results of this study indicate that friendship and prosocial behavior improve when the fun of physical activity increases during the free school year. Furthermore, a significant effect is confirmed between friendship and prosocial behavior.