In the Philippines, the recent curriculum reform has impelled the attention of various education sectors for teacher professional development (PD). The purpose of this qualitative this study is to assess the impact of year-long research-based collaborative PD of 30 elementary school science teachers with university science education researchers. The PD activity followed the lesson study model which features collaboration and reflection. Data were obtained from audio and video-taped transcripts and formal interviews. The constant comparison method was used to analyze and formulate themes from the data transcripts. Reliability of codes was ensured used the Cohen inter-rater reliability analysis with an acceptable value of 0.734. Results reveal that a change in the teachers’ professional culture was the main impact of study which encompasses collaboration, teachers’ practices focused on student learning, continuous teacher learning and reflective practice.