This paper tries to re-examine the issue of a new topic that is complements in English sentence construction in terms of Theme and Rheme. It presents an overview on the sentential complement as a universal linguistic phenomenon in natural languages across the globe. The paper adapts 'Systemic Functional Linguistics' to discuss two important linguistic concepts of discourse known as ‘Theme' and ‘Rheme' which other linguists handle them as topic of a sentence in a text (what is new) and the additional part (what is given) respectively. The study which focuses on analysing complements in terms of their Thematice and Rhematice functions discovers that complement in English sentence structure plays an important role in describing or adding new information to the other main parts of a sentence. In its analysis, this paper discovers cases in which the fronting of the complement in a theme position is taken place, highlighting the role of the context in thematizing complements in sentences. It further shoes that all kinds of complement are in Thematice rather in Rhematice position. Having analysed many examples, the paper concludes that thematizing of complement in a sentence has distinct syntactic, pragmatic, and semantic functions.