The present study aimed to address an area of language learner strategies research which has not received the same attention as reading, writing, speaking, listening, and vocabulary strategies in spite of the significant role of grammar in the conceptualization of language competence. The paucity of empirical investigations in intervention research concerning grammar strategies was the impetus to carry out a grammar strategies-based instruction on the basis of several models including Oxford's strategic self-regulated model of learning to see its effects on learners' achievement of grammar across different age groups and proficiency levels. To this end, six groups of Iranian EFL learners, comprising 63 high school and 96 university students, were chosen from intact classes in Hamedan(Iran) for control and experimental groups. The treatment focusing on comparison of adjectives and adverbs, relative clauses, articles and prepositions continued for ten sessions after the administration of proficiency tests, pre-tests and immediate and delayed post-tests. The results of ANCOVA revealed meaningful differences between control and experimental groups. However, the variables of age and proficiency contributed to the significance of differences among the groups. Generally, the treatment proved to be especially useful for youngerlearners at lower proficiency levels. In addition, the delayed effect of the treatment was quite noticeable in all experimental groups. The findings stressed the usefulness of GSBI for EFL learners, which requires that teachers, material developers and syllabus writers consider the potentiality of such a strategy instruction for the development of EFL learners' grammar proficiency.
