Dictation is of utmost importance in teaching English and Persian languages in Iran, though it is usually neglected in teaching Arabic, and no specific course is usually dedicated to dictation in Arabic. Nor does any evidence exist on the Arabic teachers’ enthusiasm for using dictation in their classes. This may deprive the students of the benefits of Arabic dictation in their language education, as these merits are beyond mere correct spelling of words. The present study, hence, tries to use a quasi-experimental method to examine the effect of Arabic dictation on the Iranian high school students’ writing ability. For this purpose, 56 first grade high school students formed the experimental and control groups of the study. To test the homogeneity of the groups before the study started, a pre-test of dictation was given to both groups. Experimental group, then, received the treatment including dictation in every session of their classroom. Control group, on the other hand, learned Arabic grammar and practiced translation as is usual in typical Arabic classes. The procedure took a complete educational year to complete. The results of the study revealed a statistically significant difference between the dictation errors of the groups. They suggested the positive effect of dictation on the improvement of the learners’ writing ability and reduction of their writing errors.