As it is worthwhile to explore the issue of how teachers’ professional development influences group cohesiveness and school organization effectiveness, this study probed into the relationships among group cohesiveness, teachers’ professional development, and organization effectiveness in elementary schools. A questionnaire survey was conducted to collect data from a sample of public elementary school teachers based on statistical data published by the Education Bureau of Kaohsiung City Government. The results indicated that group cohesiveness had a positive and significant influence on teachers’ professional development and organization effectiveness. Teachers’ professional development also had a positive and significant influence on organization effectiveness. This study further demonstrated the mediating effect of teachers’ professional development on the relationship between group cohesiveness and organization effectiveness.