This exploratory study examined how the level and nature of teaching presence impacted two online forum discussions from three dimensions: participation and interaction, cognitive presence, and knowledge development via assimilating peer messages. Effects on participation and interaction were graphically depicted. Effects on cognitive presence and knowledge construction via assimilating messages were suggested via statistical analysis, followed by qualitative interpretations. Twenty-six tertiary online learners with varied demographic backgrounds participated in the study for 6 weeks. The results showed that the nature of teaching presence in the study, specified to teacher initiation, roles of teaching presence and means of making teaching presence, largely shaped the impact of teaching presence on learning. A higher level of teaching presence was observed to be associated with a lower level of student participation, peer interaction, cognitive presence and learning uptake. Based on the results, implications for integrating and researching teaching presence in computer conferences were provided.