As task-based language teaching focuses on real word tasks and the learners need to complete these tasks in the process of learning a foreign or second language, it helps target language fluency and student confidence. That is why second and foreign language teachers and researchers have shown interest in TBLT. This study attempts to investigate the effects of employing task-based writing instruction on Iranian EFL learners' writing competence. The participants included 69 Iranian EFL learners at the intermediate level and they were placed randomly into a control group and an experimental group. The students in the experimental group performed writing tasks using task-based language teaching techniques, while those in the control group practiced writing skills using traditional writing exercises. To collect the pre-test and post-test data, the researchers administered the writing sections of two paper-based TOEFL tests and analyzed the data through Statistical Package for Social Sciences using descriptive statistics, t-test, and analysis of variance. The results showed significant improvements in the writing ability of the Iranian EFL learners who practiced writing skills using task-based language teaching techniques. Besides, using task-based writing techniques improved the Iranian EFL learners' ability significantly in terms of different aspects of the writing competence, including sentence mechanics, language use, vocabulary, content, and organization.