The current study examined the effects of different types of instructor comments on student engagement in an online discussion. In particular, this study examined three comment types: (1) praise-oriented comments, agreeing with students' initial messages and recapping their ideas, (2) elaboration-encouraging comments, requesting elaboration on the initial messages, and (3) perspective-widening comments, suggesting different or thought-provoking opinions responding to the initial messages. Seventeen graduate students enrolled in an online course were recruited. A content analysis was employed to evaluate the levels of knowledge construction. Interactivity was measured by the number of posts and idea units. The findings revealed that perspective-widening comments facilitated students' evaluation on the peer postings and brought up diverse perspectives, contributing to knowledge construction. While elaboration-encouraging comments led to high interactivity among students, praise-oriented comments had no significant effect on the frequency of interactivity.