A good English writing skill can be a ticket to university and obtain a degree in the university, or a job coupon in the society as Macao becomes an international city where a lot of people who are good at English writing are needed. However, how can we ensure that out readers get the message that we intend to give, and how do we obtain the messages from what we read or hear? In describing the semantics of a text, we are actually describing its lexico-grammatical organization. This thesis is based on the Hallidayan Systemic Functional Linguistics (SFL) theoretical framework, which is mainly aimed at investigating the mood and modality of interpersonal function of language in the students’ English writings of a Chinese Secondary school. In view of this, it first brief describes the general situation of the Macao educational system and a specific situation of a Chinese secondary school, and then presents the definition of genre, the three variables of register and the three metafunctions of language, from which it goes on to an analysis of my teaching materials with the register variables, and then a discussion of what resources the interpersonal function of SFL tools have to offer in students’ writing. The findings show that the lexical choice in the English text depended on the linguistic and cultural background of the writer. The function of a text, and the social norm and genre affect the decision of the interpersonal function of language. The change in the discourse of tenor leads to a change in the written text as well. In conclusion, the thesis sums up that SFL can help in teaching English writing in Macao as long as teachers are familiar with this theoretical framework and guidance is provided in teaching text materials. It is helpful as it is not only a set of rules like the traditional grammar, but it looks at the potential of the language system where students are assumed to be more interested in. Keywords: SFL; mood; modality; genre;register;metafunctions; interpersonal function