Effective professional development enhances teachers' instructional skills and in turn improves students' learning. This study adopts a multilevel perspective to investigate the interplay of self-directed learning (SDL), professional self-awareness, and the supportive organizational climate in terms of their effects on the professional development of high school teachers in Taiwan. The study confirms that individual SDL and a supportive organizational climate positively affect teachers' professional development. Specifically, professional self-awareness mediates the relationship between SDL and professional development at the individual level. The cross-level moderation of the supportive organizational climate is confirmed on the relationship between SDL and professional development. It is concluded that teachers with a greater degree of individual SDL and professional self-awareness, combined with a supportive organizational climate, will be more involved in professional development. Implications and suggestions for future research are also discussed.