The purpose of this paper is to examine the cause of awkwardness by analyzing sentences in college students` academic essays and classifying error types. Our analysis shows three facts. Firstly, the group of sentences that have semantic vagueness is divided into one subgroup that comes from semantic problem and the other subgroup that comes from syntactic problem, and mainly the syntactic problem in sentences brings about the semantic vagueness. Secondly, the sentences which have semantic problem are divided into two groups, one with the problem of coherence and the other with the contents errors. Thirdly, the sentences which have semantic clearness but are improper, are divided into sentences with surplus meaning and sentences which are not appropriate to the genre of text. The causes of awkwardness in sentences have to do with syntactic, semantic, textual, and contextual constraints. Syntactic problem has the highest percentage and contextual problem has the lowest percentage in corpus.