This study investigated whether formative and summative self-assessments have any differential effect on the writing performance and self- rating accuracy of Iranian EFL learners. In so doing, sixty homogenized intermediate EFL learners were assigned to two assessment conditions. After the administration of a paragraph writing pretest, the self-rating scale was introduced to the students of both groups. In the formative group, the students practiced self- assessment after each writing assignment during the term. Self- assessment error scores were, then, computed, and the students could see their accuracy extent in their self-assessment during the term. In the summative group, the students’ accuracy of self- assessment during the term was considered in their final term score. To check the students’ improvement in writing and rating accuracy, a posttest self-assessment was administered. ANCOVA results revealed under both conditions, equal improvement occurred in the students' writing. However, the summative group students improved much more significantly in self-rating accuracy.