One effective method for improving EFL learners' foreign language communication skills is to learn L2 words and use them in speaking. For this reason, researchers aim to assist learners in acquiring vocabulary to the greatest extent possible. The present study investigated the impact of EFL teacher code-switching (CS) and English-only (EO) instructions on vocabulary acquisition in EFL learners aged 10 to 13. Fifty-six early adolescent learners participated in storytelling activities under the CS or EO conditions. The key difference between the two was that, in the CS group, the teacher used the learners' L1 for teaching vocabulary. In contrast, instruction was conducted solely in English in the EO condition. Results indicated that switching to learners' L1 enhanced vocabulary acquisition more effectively than English-only instruction, and this effect persisted in a delayed post-test two weeks later. Therefore, it can be concluded that teaching new vocabulary via teacher’s resorting to the learners’ L1 through storytelling activities brought about significant gains in remembering the target words in CS condition; hence, for more effective mastery of the new vocabulary among early adolescent learners, it is suggested to present the words in learners’ first language instead of presenting them in lengthy English explanations. The study has practical implications not only for early adolescent EFL learners’ classrooms but also for other learners of different age groups to maximize their vocabulary learning and performance through teachers’ justified amount of L1 use.
