In recent years, sentiment analysis has received considerable attentions in Natural Language Processing (NLP) community (Blitzer et al., 2007; Dasgupta and Ng, 2009; Pang et al., 2002) . Polarity classification, which determine whether the sentiment expressed in a document is positive or negative, is one of the most popular tasks of sentiment analysis<cite> (Dasgupta and Ng, 2009</cite> ).