Literature on Iranian English as a Foreign Language (EFL) learners' problems in their writing skills is lacking. The present study analyzes a group of Iranian EFL students' written samples to diagnose their main area(s) of difficulty in writing argumentative essays. For this purpose, a sample of 69 argumentative essays was collected. Two raters used an analytic argumentative writing scale to score the samples. Based on the scores assigned to their written samples, majority of these students indicated excellent 'content' and 'organization' levels; competent 'task fulfillment', 'vocabulary', and 'style' levels; modest 'grammar' levels; and finally, very limited levels of 'mechanics' skills. The findings indicate the positive and promising role of English language institutes in students' writing skills. The results have pedagogical implications that will help English language instructors and program developers in Iranian English language learning-teaching institutes improve the outcome of their instruction. Further research is necessary to study the EFL writing skills of Iranian school students.