This research aimed to explore (1) the characteristics and patterns of student interactions on online discussion boards throughout a course using social network analysis (SNA) and (2) the mediating effect of social presence, as the underlying mechanism, on the relationship between these interactions and students' learning outcomes. Eighty-four college students from four classes of a single course responded to an online survey asking about their perceptions of social presence, perceived learning achievement, and course satisfaction. Students' discussion threads were analyzed using SNA. The results confirmed that social presence mediates relationships between interaction measures and learning outcomes. This research also found that SNA can be a useful tool to monitor the dynamic characteristics and patterns of online discussion interactions among students and support instructors to implement immediate and relevant instruction.