Taking a linguistic perceptive on the Holy Qur'an has opened new horizons in Qur'anic studies. Michael Halliday's theory is an approach within functionalist linguistics that has currently drawn many linguists' attention. Halliday believes that different components of speech have three ideational, interpersonal and textual metafunctions, hence his attempt at regulating three corresponding semantic layers. The present research aims to analyze the discourse of Surah Al-Falaq and Surah Al-Nas at the level of textual metafunction based on Halliday's functionalist theory. Therefore, through studying the thematic and informative structures of Surah al-Falaq and Surah Al-Nas and extracting the elements and techniques used for creating cohesion, we found that the two surahs are, in terms of discourse, on the basis of conversation and question and answer, and the use of Halliday's discourse-based components, i. e. textual metafunction, does not vary across them, but in their textual cohesion a variety of devices are effective. In addition, the two surahs differ in terms of the degree of the utilization of these cohesive (grammatical, lexical, and semantic) devices. The first and the most important cohesive device in the texts of both surahs are verbal and lexical elements, and repetition, taking different forms, is the most frequently-used device.