The main purpose of this research was to examine the relationships among school principals' transformational leadership, school academic optimism, teachers' academic optimism and teachers' professional commitment. This study conducted a questionnaire survey on 367 teachers from 20 high schools in Taiwan by random sampling, using principals' transformational leadership scale, a school academic optimism scale, teachers' academic optimism scale, and teachers' professional commitment scale. The results of confirmatory factor analysis showed that there is good fit in the factor structure of school academic optimism and teachers' academic optimism. The results of the Pearson product-moment correlation analysis and path analysis revealed that principals' transformational leadership has a positive effect on school academic optimism and teachers' academic optimism. School academic optimism has a positive effect on teachers' academic optimism. Principals' transformational leadership and teachers' academic optimism have a positive effect on teachers' professional commitment. Finally, this research offers some suggestions based on the results.