In recent times, Taiwan’s grade 1-9 science curriculum has been undergoing landmark reform that emphasizes continuity between elementary and secondary school science, integration among different science subjects, and connections between textbooks and students’ life experiences. School teachers are expected to grow professionally to meet the new challenges that have arisen from this reform. This study intends to investigate how in-service science teachers grow professionally in a collaborative professional development project based on an interactive development/improvement model operating on the principles of diversity and equal partnership. A group of science teachers and university experts with diverse backgrounds collaborated on a two-year curriculum development project. We collected and analyzed various types of qualitative data, using interviews, videotapes of discussions, and written reflections. The results show that this model succeeded in enabling teachers to grow professionally as curriculum developers and active leaders. We found that diversity and equal partnership were the key factors responsible for the success of this project, and believe the two factors should be taken into account when creating teacher professional development programs.