The purpose of this paper is to explore and compare learners' critical thinking and interaction during an asynchronous online discussion when peer- or instructor-facilitation was provided. Current literature on online discussion reveals a controversy between peer facilitation and instructor facilitation regarding their strengths and weaknesses. However, the effect of peer-facilitation on critical thinking learning outcome has not been clearly discussed. Situated in a graduate-level program evaluation course, the learners engaged in a debate using a scenario-based case on ethical decision-making. A content analysis of discussion using the Cognitive Presence framework and a social network analysis revealed a significant difference between peer-redirected group and instructor-redirected group in their cognitive presence as well as in interaction dynamic upon receiving the redirection message. Based on findings regarding cognitive presence level, interaction dynamic and perspective change on the debate topic in each group, a peer-facilitation approach is more effective for fostering critical thinking and collaborative discourse.