Professional development of in-service teachers requires a sustainable and school-based model which will provide them with opportunities for collaboration in building an environment that supports their capacity-building and pedagogical improvement. In this professional development model, teachers are treated as central to the learning process, allowing them to be engaged in critically assessing their own improvement. A qualitative method was used in this study to explore the positive outcomes of lesson study as a professional development model. Data included transcripts from year-long interaction of the teachers and science education experts taken in various form such as reflective logs, audio and video-tape recordings, non-structured questions and formal written interviews. Analysis was done through the constant comparison method to reveal the coding patterns for the positive impact of lesson study. Teachers' insights provided positive evidence that lesson study developed them professionally, especially in terms of enhancing their science content knowledge, improving their teaching strategies aligned to inquiry, building a collaborative and professional working environment, and recognizing the significance of post-lesson reflection and discussion. Results therefore show the growing acknowledgement of lesson study as a professional development model for in-service teachers which can possibly be adapted by other Asian and international education settings.