In the late 1970s and 1980s, Discourse Analysis was greatly influenced by a number of studies. Halliday emphasized the social functions of language. In Britain, Sinclair and Coulhard developed a model for the description of Teacher-Pupil talk; other similar works have dealt with Doctor-Patient interaction, interviews, debates and so on. Meanwhile, in America, the work of Goffman, Sack and Jefferson is important in the study of conversation, turn-taking, and other aspects of spoken interactions. Thus, Discourse Analysis is a rapidly expanding field, providing insights into various aspects of language in use and therefore of great importance to language teaching. Traditionally, language teaching has dealt with pronunciation, grammar and vocabulary; but now it is Discourse Analysis that raises our awareness of how to put this knowledge into action to gain successful communication. Business letters in general and letters of enquiry in particular have long been considered as key documents in the business context due to the fact that Vietnam nowadays is step by step adhering to the development in the world. Consequently, we have joined a lot of international organizations and corporations; we also have signed international documents particularly in the business transactions with other countries. Among those documents and texts, business correspondence plays a key role. In fact, writing business correspondence is becoming a more and more important task in many corporations and companies. The letter of enquiry is indeed significant among various kinds of business letters thanks to its frequency in use. So many factors have to be taken into consideration in the process of writing a letter of enquiry; namely the format, the style, the language, so on and so forth. Additionally, the knowledge of cohesion and coherence is greatly essential in discourse construction and comprehension for communication. Cohesion and coherence are actually regarded as the important aspects of language usage.