This study wanted to understand the fossilized and developmental grammatical errors in Persian EFL learners’ compositions at three levels of proficiency. The researchers divided the participants into lower-intermediate, upper-intermediate, and advanced levels. The researchers collected three compositions from each participant. After identifying the grammatical errors for each level, the researchers compared their frequency and mean differences across these levels to identify developmental and fossilized errors. Finally, using SPSS, five grammatical error types were identified, of which the most frequently made was the wrong selection of verb forms. Out of 5 error types, the researchers identified one as developmental (verb forms) and four as fossilized (tenses, prepositions, articles, and active and passive voices). Findings indicate that most students committed fossilized errors, showing the permanent retention of grammatical errors in their compositions. Accordingly, serious pedagogical considerations need to be given to the teaching of grammar to improve the writing competence of Iranian EFL learners. Manuscript profile