Part of speech category is a basic task in the study of grammar, and has been considered to be a long-held challenge in grammatical analyses. For the part of speech category in Chinese, an overall consensus has been reached in the academic circles through long years of researches and discussions that ,with semantic criteria basically discarded earlier on, part of speech category in Chinese should be analyzed with functional criteria instead of morphological criteria which are associated with entanglement. While this application of functional criteria seems to solve the difficult problems with the part of speech category in Chinese, it is still open to question whether this application should be viewed as a logical and reasonable approach. Most words in all the languages have the possibility of syntactic combinations, but it is an undeniable fact that there are definitely certain words used in the pragmatic expressions which are rarely subjected to syntactic restrictions. When these words with pragmatic functions are analyzed by the criteria of syntactic functions, they can have no places in the existing part of speech category in Chinese. For instance, words such as “例如、如、比如” in Chinese, which represent the meaning of exemplification, do not fit into any grammatical function of part of speech category. As a new potential for the functions of these words has been presented in the studies which are associated with discourse marks in the Western linguistics circles, The word “例如” in Chinese should be regarded as a discourse mark of exemplification function, and words like “真的、说句实话” as discourse marks which represent facts. These kinds of words should not be analyzed from the perspective of traditional syntactic functions because they obviously represent pragmatic functions. Therefore, part of speech category should be first divided into two classes based on the criteria of grammatical function of words: words with syntactic functions and words with pragmatic functions. Although words with pragmatic functions are greatly outnumbered by words with syntactic functions, they have their own particular grammatical functions and thus should be exclusively studied in accordance with their pragmatic properties, independently from the existing criteria of part of speech category.