The present study explored Iranian EFL learners’ language learning strategy use and the impact of language learning strategy instruction on their L2 achievement. To this end, 1608 junior and senior EFL students participated in the study by completing Strategy Inventory for Language Learning (SILL) developed by Oxford (1990). Also, 30 participants, selected from among the participants of the study, sat a semi-structured interview. Moreover, 40 less successful EFL learners were selected to take part in the experimental phase of the study. The results obtained from descriptive statistics revealed a) Iranian EFL learners tended to adopt cognitive strategies most and compensation and memory strategies least, b) successful learners preferred metacognitive strategies most and cognitive strategies least, c) less successful learners used cognitive strategies most and memory and compensation strategies least, d) cognitive and compensation strategies were found to be the most and the least frequently used sub-categories of language learning strategies by all the participants of the study respectively, and e) Iranian EFL learners, overall, and successful learners were both medium strategy users. However, the less successful participants reported using language learning strategies with a relatively lower frequency. Moreover, the results of chi-square analyses indicated that there existed a significant difference between successful and less successful learners on their frequency of use of language learning strategies. Furthermore, the results of two Independent Samples t-tests and two separate Paired Samples t-tests showed that explicit strategy instruction had a significant positive effect on L2 achievement of less successful Iranian EFL learners as the participants in the experimental group significantly outperformed their counterparts in the control group.
