This study attempts to verify the relationships among principal's transformational leadership, leader-member exchange (LMX) quality between the principal and teachers, and teachers' organizational commitment which are perceived by junior high school teachers. The methodology is the questionnaire survey and the subjects are 279 formal engaged teachers of public junior high schools in Taichung County. The results indicate that there is no significant positive impact of the principal's transformational leadership perceived by junior high school teachers on teachers' organizational commitment whereas there is a significant indirect effect. Moreover, there is a complete mediation effect of LMX quality for the impact of principal's transformational leadership on teachers' organizational commitment; that is, the higher perceptions of junior high school teachers for principal's transformational leadership promote their LMX quality with the principal, and further advance teachers' organizational commitment to schools. Finally, based on key conclusions, this study interprets the implications of junior high school principals' transformational leadership and proposes some suggestions.