This study investigated how a collaborative mentoring program influenced beginning science teachers' inquiry-based teaching and their reflection on practice. The one-year program consisted of five one-on-one mentoring meetings, weekly science education seminars, weekly mentoring group discussions, and self-evaluation activities. The participants were three beginning science teachers and three mentors at the middle school level (7-9th grades) in an urban area of South Korea. For each beginning teacher, five lessons were evaluated in terms of lesson design/implementation, procedural knowledge, and classroom culture by using the Reformed Teaching Observation Protocol. Five aspects of the beginning teachers' reflections were identified. This study showed that a collaborative mentoring program focusing on inquiry-based science teaching encouraged the beginning teachers to reflect on their own perceptions and teaching practice in terms of inquiry-based science teaching, which led to changes in their teaching practice. This study also highlighted the importance of collaborative interactions between the mentors and the beginning teachers during the mentoring process. (Contains 1 figure and 4 tables.)