Writing as one of the important skills needed for communication has always been the topic of research in English teaching contexts. To improve the writing of English students, teachers employ different methods. This study was designed to investigate the impact of the inquiry learning method on the grammatical accuracy of Iranian EFL learners’ writing. To do so, 90 intermediate EFL learners from Abadan Medical University were selected through the Oxford Placement Test (OPT) and participated in a quasi-experimental study, and then they were randomly divided into experimental and control groups (EG & CG). Their ages ranged between 20 to 30. A pretest was administered to compare the results of their performance at the beginning of the study. The inquiry learning method was taught to the EG as the treatment, while the conventional method was employed for the CG. In the experimental class, the teacher did not teach everything directly, and the learners were expected to discover the knowledge and generate rules, but the presentation of materials in the CG was conventional without any discussion, and the students in the CG were mainly passive. After the instruction, a posttest was given to both groups to observe the impact of the inquiry learning method on the grammatical accuracy of their writings. The analysis of the data was accomplished through the application of a dependent sample t -test and an independent-samples t- test to compare the mean scores, and the results indicated that the inquiry learning method improved the EGs̕ writing, and they outperformed the CG. The findings of this study could have implications for the EFL learners, teachers, and material designers that may help them in EFL language learning and teaching.