Acquiring writing skills is more difficult compared to other three skills because writing skills require mastery of many different linguistic elements of the language that are beyond what is just taught in the standard curriculum. Therefore, EFL teachers should adopt effective strategies and techniques to develop the students' proficiency to write well. This study was conducted to explore the effectiveness of genre-based instruction as a teaching strategy on the Iranian middle school students' proficiency in argumentative writing. To do so, a mixed method design was adopted in which 40 EFL students were selected based on a multi stage sampling procedure form high schools in Varamin, Iran. A researcher-made writing test and a perception questionnaire were used to collect the quantitative data. Cronbach's alpha was used to check the internal consistency of the questionnaire. The participants were grouped into genre-based instruction class (GBIC) and control class (CC). Before they began the writing lessons, the students were given a pretest so their initial writing ability would be similar. After undergoing the 12-session treatment, they were given a writing assessment to gauge if the treatment had been effective. Additionally, the perception questionnaire was administered to collect data on students' attitudes towards the genre-based instruction (GBI). This study also collected qualitative data through an interview and in-classroom observation. The experimental findings of this current study revealed that GBI had a significant effect on the development of argumentative writing ability among EFL students. تفاصيل المقالة