The current study aims to add to the debate on whether or not the use of L2 learners’ mother tongue should be allowed in the classroom by evaluating the efficacy of specific instruction types. The current project was administered in two Iranian EFL classrooms to determine the differential effects of teachers’ first language (L1) use and second language (L2)-only explanations on Persian-speaking adults’ acquisition and learning of concrete and abstract English words. A sample of 30 Low Intermediate and Intermediate level EFL learners was chosen for L1-use group; 30 participants were allocated to an L2-only use condition, and another 30 participants to a comparison group. Pretests, immediate posttests, and delayed posttests (four weeks later) in the form of the revised vocabulary knowledge scale (RVKS) were applied to all participants. The study found that teacher's code-switching can result in higher vocabulary improvements in immediate and delayed retention, notably for concrete words in the process of teaching and learning.