Lesson study provides an opportunity for teachers to improve their knowledge and skills of classroom teaching, where a group of teachers collaboratively research, plan, and observe a series of lessons followed by ongoing discussions and reflections. While social interaction between group members plays an important role in collaborative learning contexts, there are concerns about the quality of interaction among teachers in a lesson study (e.g. inadequate engagement in discussion, and the process of interaction dominated by a few politically strong teachers). To address these issues, this study proposed a visualization-based group awareness tool that visualized individual and group behaviors and performance as well as peer comments to facilitate group interaction in a lesson study implemented on an online platform. Twenty-four primary school teachers participated in the study, and their interaction behaviors and lesson plans generated during the study were collected for analysis. The results revealed positive effects of the visualization-based group awareness tool on enhancing teachers' engagement in social interaction and fostering coherent interaction among members during the lesson study, and improving teachers' performance in learning process design through the study.