The rise of the microblog in China, and in the world, introduces a critical issue of knowledge production that has not yet received enough attention. This research explores the differential knowledge production in the Chinese micro blogosphere through a case study of Sina Weibo users' discussion about nuclear power during Japan's nuclear crisis in 2011. Quantitative and qualitative analyses of related Weibo messages reveal a knowledge production gap even in this relatively open social network with its low barrier of knowledge production. While knowledge production on the Weibo platform is more grassroots-oriented, traditionally dominant social groups are playing a more significant role in the knowledge production process. They not only produce knowledge in a more active and rational way, but they also generate greater social influence in the micro blogosphere. The theoretical, methodological, and practical implications are discussed.