The purpose of the present study is to find out the features of English business letters. Halliday's systemic functional linguistics is used as the theoretical framework, mainly, interpersonal fucntion. The English business letter (EBL) is an important written text used for international business communication and it has its own features of text. This paper explores the interpersonal function of EBL in terms of mood, modality, interaction and negotiation and finds that EBL has its unique interpersonal features.