This study aimed to investigate the effect of role-play through dialogues vs. written practice on Iranian intermediate EFL learners’ knowledge of English idioms. The question this study tried to answer is if role-play through dialogues vs. written practice has a significant effect on Iranian intermediate EFL learners’ knowledge of English idioms. To find the answer to the question, 30 intermediate level students from Iranian institute were selected. They were both male and female. Intermediate level students who were dealt for the current study were selected because they were learning all skills of language at the same time, they were familiar with words, and their English proficiency was enough to speak English and understand the importance of speaking. The students were selected administrating OPT which divided into two groups, one experimental group (Role-Play through Dialogues) and one control group (Written Practice). The results of this study revealed that the experimental participants performed better on learning idiom.