The main aim of this research is to figure out the similarities and differences between English and Vietnamese business complaint letters, which is regarded as one of the most popular kinds of business correspondence, in terms of the four aspects of Systemic Functional Grammar namely transitivity, mood, theme and cohesion. To fulfill this objective, the authors conducted a survey with a sample of 30 complaint letters, including fifteen letters written in Vietnamese and fifteen letters written in English. The research results indicate that there are no significant differences in terms of the four aspects of Systemic Functional Grammar used in English and Vietnamese business complaint letters. However, in terms of cohesion, the results show that while Vietnamese writers regard lexical devices as the most useful tools in their letters, English ones highly appreciate the effectiveness of reference. The findings of this study can be utilized as a referential material for Vietnamese learners and teachers in writing and teaching writing effective letters in English, especially business complaints.