An online discussion facilitates students' higher order thinking in online classes, especially when adopted with the instructor's guidance. The current experimental study examined the effects of two different discussion representation tools (social and cognitive diagrams) on students' discussion behaviors. The social diagram emphasized interactivity of participants by illustrating who posted messages and who replied. The cognitive diagram described how discussion topic evolved by summarizing main topics discussed. Thirteen graduate students enrolled in an online graduate course participated in the study. While analyzing five instructional cases, students were asked to discuss each case in a group that the instructor assigned randomly. For each case, one group was provided with the social diagram whereas the other with the cognitive diagram. Major findings revealed both tools facilitated online discussion activities as the instructor intended: the social diagram turned out to promote socially desirable responses while the cognitive diagram produced more cognitively desirable responses. Further studies on how the two types of discussion diagrams can be integrated in online discussions will be needed.