This study characterized students' online collaborative discourse from a theory-building perspective and examined its relation to epistemic and conceptual understanding. Fifty-two fifth graders' Knowledge Forum discussions on electricity were analysed. Discourse moves were coded within the inquiry threads, and two key epistemic patterns were identified: problem-centred uptake and theory-building moves. Analysis showed that higher-quality discourse threads included more problem-centred uptake moves in which ideas were built more coherently on each other to address the central problem. There were also more theory-building moves on explanation and sustain inquiry. We also examined the relationship between discourse moves and conceptual-epistemic understanding. Regression analyses showed that problem-centred uptake predicted epistemic cognition beyond prior epistemic cognition and that theory-building moves on explanation predicted students' conceptual understanding beyond their prior science understanding. Implications for fostering more productive discourse and sophisticated epistemic cognitions using online discussion are discussed.