Social Network Analysis, Statistical Analysis, Content Analysis and other research methods were used to research online learning communities at Capital Normal University, Beijing. Analysis of the two online courses resulted in the following conclusions: (1) Social networks of the two online courses form typical core-periphery structures; (2) Social networks of the two online courses contain "structural holes," where some actors position themselves to become potential opinion-leaders within their social networks; (3) Actors, variously positioned within a core-periphery structure, show quite significant differences in terms of knowledge building; (4) Taking "structural holes" into account, there exist considerable differences in knowledge building between opinion-leaders and non opinion-leaders; (5) Actors in the "core" and "structural hole" positions have very different characteristics in terms of knowledge building. These actors in particular play important roles in online learning communities, impacting on the level of the constructed knowledge.