This article aims at analysing how Systemic Functional Linguistics’ interpersonal metafunction can inform us about the notion of power relations expressed in both the original English version and its Thai translation in a case study of the popular contemporary novel Fifty Shades of Grey written by E.L. James in 2011. The study analyses conversations between the two main characters, Christian Grey and Anastasia Steele, taking place during their intimate acts which clearly demonstrate the two main characters’ power relations. Systemic Functional Linguistics’ interpersonal metafunction is concerned with the social role relationship between text participants and is applied as an analytical tool in order to see how it can explain the notion of power relations when one communicative participant has more power than the other. Analysing the mood structure of lexico-grammar (Eggins, 2004) at the textual level has found two types of clause, imperative and declarative, that are used to construct the notion of power relations between the two characters. Examining interpersonal metafunction’s tenor regarding power, contact and affective involvement further explains the notion of power relations occurring on the contextual plane. Although the findings demonstrate a certain degree of difference in the representation of power in the Thai translated version of the novel, that is, in the use of final particles; it is a characteristic of the Thai language that does not exist in English.