The present study aimed at investigating the effect of weekly quizzes on Iranian high school students' performance on final achievement tests. This effect, which is technically referred to as washback, is defined as the effect of testing on learning and teaching. Most scholars have attempted to provide guidelines in order to achieve positive washback. While some suggest frequent quizzes as a means of positive washback, others oppose the idea. The proponents claim that frequent quizzes stimulate practice and review, give the students more opportunities for feedback and have a positive influence on students' study time. The opponents, on the other hand, believe that too frequent testing might inhibit of larger units of instructional material and frustrate anxious students, and thus, hinder learning. This study explored whether students taking weekly quizzes performed better in the final achievement tests. 70 students studying in grade two of high school were selected and divided into two groups. One group received weekly quizzes and the other group took only midterm exam. The results indicated that the group with weekly quizzes performed better than the group without quizzes.