Öz Problem Statement: Relativization is an important grammatical sub-system for second language learners. This study intended to explore the effects of different types of L2 instruction on the learning of English relative clauses by Persian learners.Purpose: The differential effects of the three types of treatment (i.e., Focus on FormS, Focus on Meaning, Focus on Form) on the learning of English relativization was investigated.Methods: Intact university classes of English learners were divided into three groups receiving different forms of instruction. Accuracy of the target form was measured by two distinct tasks of sentence combining test and grammaticality judgment test.Findings and Results: The results of the two tests show improvement of all three groups, the focus on form treatment group outperformed the other two on both tests, however. This study also suggests that learners’ attention to detailed analysis of form facilitates the learning of relative clauses in this context.