With the development of global economy, more and more business negotiations are proceeding in which conflicts and disputes are inevitable. Business negotiations involve interpersonal relationships, which is usually expressed by mood and mo-dality. Modal verbs, as an essential approach for expressing politeness with rich meanings of modality, are widely employed in business negotiations. This paper focuses on analyzing the polite usages of modal verbs in business negotiations by Politeness Principle. Based on this theory, three interpersonal functions expressed by modal verbs are to be studied:Cost-and-Benefit, Humor and Choice.