Idioms feature prominently in daily communication. Accordingly, teaching and learning idioms should be a primary concern in language education, including English education. However, there is relatively little research on the role of formal instruction of idioms in developing idiomatic competence. This study investigated the instructional effects teaching idioms in four modes (short movie clip, sentence-use, definition, and role-play) had on learners' idiomatic competence. An idioms test was distributed among 47 English language learners at two language institutes to establish their idiomatic knowledge prior to the experiment. One hundred idioms were taught to all participants in the course of one month. The one-way analysis of variance (ANOVA) of the posttest results revealed significant differences among the four idiom-teaching modes. Discussions and pedagogical implications are provided in the paper.