Mobile instant messengers such as WhatsApp have become an essential part of everyday life and can be a transformative tool for teacher professional development. Several studies have addressed the use of WhatsApp in a community of practices. However, there is a lack of study on examining teachers' interaction in WhatsApp community of practices, particularly using social network analysis (SNA). This study aimed to investigate teachers' interactions with emphasis on frequency and pattern of interactions in eight groups of WhatsApp community of practices, which were voluntarily formed. The participants of this study were 76 secondary school mathematics teachers in Banda Aceh, Indonesia. Data were collected during the two months of the program and were analyzed using descriptive quantitative analysis and SNA. This study reveals that there were great differences in teachers' interaction across the groups. Participants were predominantly involved in discussions during evenings, either weekdays or weekends. In addition, this study indicates that the most active participants in each group, who played a critical role, sent the highest number of messages and interacted with most of the other members of their groups. The participants also started a new session of discussion after a short or long activity. The findings of this study could be the basis for sustainable professional development using mobile instant messenger technology in the context of the community of practices.