The purpose of this study was to test the mediated effect of school sports club participation in relation among physical self-concept, sociality, and school happiness. The participants were 243 middle school students. All the students administered three instruments, physical self-concept scale, sociality development scale, and school happiness scale. The results of this study were as follows: First, there are significient relationship among physical self-concept, sociality, and school happiness. Second, it was proved that sociality exerted as mediating variable between physical self-concept and school happiness. The implications of this study were investigated variables regarding school adjustment to help middle school students. The implication of this study is that it revealed the structural relationship of the variables related to the school happiness of the participants in the school basketball sports club. Based on the results of this study, basic data were provided for the establishment of school systems and the development of curriculum for the promotion of school happiness for students in the future.