The objectives of the present study were threefold: First, it sought to investigate difficulties Iranian EFL high school learners face to understand English idioms. Second, it attempted to explore intermediate EFL learners’ language learning strategy preferences to comprehend idioms. Third, it aimed to examine whether there was a significant difference between successful and less successful students' strategy use in idiom comprehension. To this end, 200 Iranian male and female students of the three grades of different senior high schools in Qom, Iran, were selected through convenience sampling. They responded to a five-point Likert-scale questionnaire (Alhaysony, 2017), investigating the challenges of idiom comprehension. Afterwards, they took the Oxford Quick Placement Test. Ninety-eight of them were selected as intermediate learners. Third, they took a multiple-choice test on idiom comprehension, developed by the researcher, and responded to the Strategy Inventory for Language Learning (Oxford, 1990). Descriptive statistics, frequencies and means, and a one-sample t-test were used to analyze the data. The results indicated that the participants complained that idioms were challenging mainly because they were not taught well in class and were not part of the course syllabi. They also revealed that successful learners in understanding idioms used all six categories of strategies in the high and medium levels. They employed metacognitive strategies the most (M= 3.88), but affective strategies the least (M= 2.84). The results of the t-test suggested significant differences between successful and less successful learners' strategy use. The results provide insights into the challenges and strategies of English idiom comprehension.
