Online discussion transitions were analysed as a means of furthering the development of critical thinking disposition of students and their attitude toward disaster mitigation. The discussion was organised as an additional learning activity during a fully online course. The participant's posted messages were analysed using a network analysis technique in order to promote discussion and learning performance. The posting chain was represented as an adjacency matrix and posting relationships were analysed. The results suggest that discussant’s activity shows a hierarchical structure, with some participants posting in response to lecturer's instructions. Posting participants were classified into several clusters, and these characteristics were analysed. Though single-posting participants were not active discussants, their characteristics, including their learning performance, could be readily recognised correctly during the analysis.