This study investigates whether L2 learners of English rely on their L1 (in this case Chinese) when attempting to comprehend an idiom in English and whether the student's level of proficiency has any effect on L1 use in understanding L2 idioms. Using think-aloud protocol, the following conclusions were reached: 1) Almost all subjects translated English idioms into Chinese and searched a match, i.e. there exist transfer and interference of L1 in understanding L2 idioms.2) When the L1 was unable to help for understanding different idioms, good students were able to use conceptual strategies (i.e. metaphors and schema) better than poor students. This paper concludes with a suggestion that metaphor instruction be used to teach L2 idioms since a student cannot get very far when relying solely on his L1.