Discourse relations may be either mononuclear or multi-nuclear. A mononuclear relation holds a nucleus and a satellite, the nucleus usually reflects the intention focus of the discourse, while the satellite represents supportive information. Although there are several previous works focused on analyzing discourse relations, only limited works focused on recognizing nuclearity between discourse units. In this paper, we propose a discourse unit nuclearity classifier in Chinese Discourse Treebank (CDTB). Our classifier considers the context of the two arguments, part of speech information, as well as word pair information as the features. The evaluations in CDTB corpus shows the effectiveness of our methodology and creates a baseline of 53.21% in accuracy.