Preliminary efforts to examine teachers' online participation and depth of messages have tended to focus solely on message metrics (e.g., counting the number of messages). However, such efforts do not address online messages in relation to one another. Taking relations of messages into account is important as the messages are not individual online postings but are a representation of teachers' interaction. Thus, this study proposes a research methodology-social network analysis to address the relations of teachers' online messages. It focuses on teachers' communication patterns and thinking levels observed in an online forum. The online forum to be examined in the current study is part of a professional development program. K-12 teachers involved in the program used an online forum to discuss their history-related curriculum implementations. The study also presents data from interviews with forum users to help validate findings attained through examining the online messages. The results indicated that faculty members' dominance in the online discussion and teachers' perceptions of more knowledgeable others adversely influenced their communication and thinking levels. The findings and future research were discussed regarding the communication patterns in online professional development programs. (Contains 4 tables and 5 figures.)