Abstarct: Flipped classroom model emerged due to the advancement of technology to facilitate the process of learning in general and writing in particular. This quantitative study investigated the effect of the flipped classroom model on enhancing the IELTS writing performance of advanced learners in Iran by utilizing a time-series design. The research involved a total of 60 advanced learners, randomly assigned to either a control group or an experimental group. The flipped classroom model was implemented with the experimental group, by using pre-class instructional materials delivered through the MOODLE application. The control group, on the other hand, adhered to conventional teaching methods. The research design consisted of pretest, immediate posttest, and delayed posttest, which were conducted over the course of the study. The pretest examined learners’ baxseline writing skills, while the posttest evaluated the outcomes of the instructional interventions. However, the delayed posttest was administered two weeks after the instructional phase, which aimed to measure the enduring impact of the flipped classroom model. The results revealed a substantial improvement in the IELTS writing performance of the experimental group. More specifically, the delayed posttest demonstrated a significant improvement in the experimental group's writing scores compared to the control group. In terms of writing IELTS scoring criteria, lexical resources along with cohesion and coherence was found to be homogeneous at these three time points. However, significant differences were observed regarding grammar at these time points, including between pretest and posttest, between pretest and delayed posttest, and between posttest and delayed posttest, Task achievement, on the other hand, was significantly different between just the pretest and posttest. Generally, this study suggested that the flipped classroom model, in line with the premises of Bloom's taxonomy (1994), facilitated a lasting positive effect on learners' writing abilities.