For the past three decades or so, language teachers, researchers and educators have been working on language learning strategies. From early examples of research such as the studies carried out by Rubin (I 975) and Stern (1975), to taxonomies of strategies like that drawn up by Oxford (1990), to theories of language acquisition which incorporate strategies (O'Malley & Chamot, 1990), much work has been done to identify what might be good language learning strategies. A major problem in measuring the learning strategies of learners of English as a foreign language (EFL) is the development of a valid and reliable instrument. This article deals with Oxford's Strategy Inventory for Language Learning (SILL). In fact, the present study investigate the reliability and construct validity Oxford's 50-item questionnaire designed to be used in foreign language settings. Five hundred and sixty eight Iranian BS and BA students took the questionnaire. The results revealed that the SILL has acceptable reliability, but does not enjoy satisfactory construct validity.