This article explores the EFL teenage learners’ knowledge of common English idioms in a Vietnamese context. Seventy-six teenage students between 14 to 17 years old, including 33 males and 43 females participated in an idiom test. The test consisted of 50 idioms selected from common English proficiency test books and the lists of common idioms by Grant (2007) and Liu (2003). Descriptive statistics showed that the students had very limited idiomatic knowledge. This finding uncovers the needs for idiom instruction. Based on the literature, the current paper proposes an instructional direction for improving their knowledge of idioms.