Open AccessThe present study investigated the role of first language (L1) transfer in Iranian EFL learners' second language (L2) idiom comprehension. It was also sought to understand whether there is any significant difference between learners of different proficiency levels and their use of L1 in decoding L2 idioms. To do this, the participants of different levels of language proficiency were asked to participate in this study. The L2 idioms were categorized based on their similarity to L1 into three groups of identical, similar and different. A think-aloud protocol analysis was performed and participants were asked to verbalize their thoughts as they read the target idioms in order to detect the strategies they used. The results showed that the most favoured strategy used by learners of different levels was translation. Translation to L1 (Persian) was also the most-frequent strategy in decoding similar, identical and different types of idioms. It was also revealed that generally the participants of different levels were significantly different from each other in using strategies.