The study examined the relationship between idiom familiarity, knowledge of idiom meaning and idiom transparency judgments in L2. A group of 23 intermediate Japanese learners of English were asked to provide familiarity ratings, transparency judgments, and definitions for 30 English idioms, 27 of which had semantically equivalent but compositionally different idiomatic counterparts in Japanese and 3 phrases for which semantic equivalents in L1 also shared the same structural properties. Transparency ratings were repeated after the instructional treatment. A comparison of pre-treatment and post-treatment transparency scores showed that knowledge of conventional idiom meanings had a strong effect on the learners’ perceptions of idiom transparency. Transparency judgments, however, were not found to be a reliable predictor of the learners’ ability to infer figurative meanings of the idiomatic phrases. Idiom familiarity was not found to have a significant effect on idiom comprehension or on transparency judgments either. A limited positive effect of language transfer on L2 idiom comprehension and transparency ratings was observed.