Knowing how to think about the language learning process is conducive to effective learning especially in the case of grammar structures. Metacognition, a concept derived from cognitive psychology, can assist learners in this case. With regard to young adolescents at lower proficiency levels, there is a long-standing debate on whether to use L1 for teaching metacognitive strategies since learners might not be efficient enough in terms of understanding the concept of metacognition. Therefore, this small-scale experimental study focused on the effect of grammar instruction through the metacognitive intervention delivered in L1 on the EFL pre-intermediate learners' grammatical performance in English and investigated their attitudes towards grammar instruction delivered in L1. To fulfill these purposes, a homogenous group of 20 language learners was randomly divided into an experimental group ( n =10 ), which received grammar instruction through the metacognitive intervention delivered in L1, and a control group ( n =10 ), which received the instruction only in English without grammar instruction through the metacognitive intervention delivered in L1. The results of the independent samples t -test indicated that the experimental group did not outperform the control group in terms of all four English grammar points, namely present perfect, simple past tense, comparative and superlative adjectives, and past progressive . In addition, the findings obtained from the five-point Likert scale questionnaire which was distributed among the learners suggested that they had positive attitudes towards the use of L1 (Persian) in teaching English grammar. We argue that low working memory capacity, overreliance on translation, and less-cognitively activated L2 processes may account for the low performance of the experimental group learners in the grammar tests. The implication is that the L1 use in an L2 classroom can be helpful in teaching L2 grammar through the metacognitive intervention, but future studies need to examine how and to what extent L1 should be used to yield more effective results in the case of lower-proficiency learners in an EFL context.
