According to the literature, flipped teaching is a relatively new pedagogical approach in which the typical activities of classroom lectures followed by homework in common teaching practice are reversed in order, and most often integrated or supplemented with some types of instructional materials, such as instructional videos or PowerPoint files. This experimental study, using a pre-test-treatment-posttest-delayed posttest design, was aimed at investigating the effect of flipped instruction on Iranian EFL learners' quality of argumentative essays. The participants were 55 students, who were assigned to two groups: the flipped classroom (FC) and the traditional classroom (TC). Each group received 3 sessions of treatment. First, whether there was any significant difference between the FC and TC in the overall quality of the essays was investigated. The FC group significantly outperformed the TC one. Then, whether the difference between the groups varied over time was explored, and it was revealed that the FC was still significantly superior over the TC. Next, whether there would be any significant change in the FC in the long run was examined, and no significant change was seen. The promising results found in FC group can be attributed to not only the flipped instruction but also the process of actively engaging the learners in their learning in addition to incorporating different techniques, such as the video screencasting, collaborative writing, as well as in-class teacher-learner interaction and negotiation because it is argued that the crucial point in flipped instruction is how teachers best use in-class-time with students.