Errors are one of the enigmatic parts in the process of foreign language (L2) learning as they are extremely versatile at each and every stage of the language learning proficiency. The present study, therefore, was an attempt to reveal Iranian EFL learners’ grammatical errors in writing at two levels of proficiency, namely lower intermediate and advanced, and then to investigate whether there was a relationship between the levels of EFL learners’ proficiency and the types of grammatical errors they committed in their scripts. The study was carried out at a private language institute in Gorgan, Iran. To this end, 60 female EFL learners (30 lower intermediate and 30 advanced females) whose age ranged between 13 and 17 participated in this study and wrote 150 word writing samples on a predetermined descriptive subject. The descriptive analyses of the data based on the scoring framework of the study demonstrated the types of grammatical errors at each level of proficiency. A Chi Square test was then run in SPSS Ver. 25 on the 16 common frequent error categories between both levels of proficiency, which verified the existence of a relationship between EFL learners’ levels of proficiency and the types of written grammatical errors they committed in their writing. The findings of the study might be of interest to EFL learners, EFL teachers, syllabus designers, and materials developers.
