Focus on form instruction is a kind of instruction that draws students, attention to linguistic elements as they arise incidentally in meaning based instruction. There are different types of focus on form instruction. The present study was designed to investigate the effect of incidental focus on form on grammatical accuracy among Iranian L2 learners. Eighty learners from Sahand language Institute in Miandoab after taking grammatical judgment test which was administered to homogenize them, were placed in two control and experimental groups. Learners in experimental group received feedback through recasting during retelling the reading passage according to principles of Jigsaw task. But learners in control group did not receive any feedback. After treatment, which lasted for eight sessions, post-test was given to both control and experimental groups to observe the difference resulted from the treatment. To be sure about the significance of the difference between post-test means of both groups, a t-test was used. The results at the end supported the hypotheses of the study and positive effect of incidental focus on form on grammatical accuracy of L2 learners. After that, for the purpose of analyzing the effect of incidental focus on form on accuracy of pronouns, tenses, articles and propositions separately, other tests (pronoun, article, tense, proposition tests) was given to the learners in both control and experimental groups. The data collected was computed through t-test which revealed that the effect of incidental focus on form on grammatical accuracy of articles is greater than pronouns and tenses but incidental focus on form didn't have any effect on accuracy of propositions. Pedagogical implications have been discussed.