The present study attempts to report a collaborative cycle of professional development in teaching elementary school mathematics through lesson study. It explores a practice of lesson study conducted by teachers aiming to improve their knowledge of pedagogy. The study adopts an ethnographic approach to examine how collaborative teaching within an adapted lesson study framework might change the teaching-learning process. More explicitly, the present research looks at how lesson study influences teaching mathematics and how it helps the teachers learn from their peers in a discursive school-based setting. The study suggests that teachers need to aim high when dealing with students, and use more daily life situations in their math problems. It also particularly reveals that lesson study could have the potential to help teachers promote their teaching and boost students' learning. Furthermore, it might also be used as an effective alternative to traditional professional development programs.