Vagueness is one of the basic attributes of natural language. This is the same to advertising language. Vague language is a subject of increasing interest, and both foreign and domestic studies have attained success in it. Nevertheless, the study on the application of vague language in the context of English commercial advertising is relatively sparse. Since the effectiveness of communication of commercial advertisements is one of the greatest concerns of advertisers, this paper is to demonstrate the functions of vague language in commercial advertising, which is a communicative factor in the effectiveness of advertisements, through analysis of vagueness in advertisements under the guidance of pragmatics, Grice's Cooperative Principle and Conversational Implicature in particular. The paper shows that vague language in commercial advertising plays both positive and negative roles. Its positive functions include improving the flexibility of communication, enhancing the persuasiveness of communication and ensuring the accuracy of information whereas its negative functions cover misleading readers and making them subject to false understanding.