The present study aimed at investigating the effects of learning lexical chunks on the speaking fluency of EFL Iranian learners (between the ages of 13 and 17) and at the same time taking into consideration the examining participants' use of lexical chunks and their knowledge of them. To this end, after administrating the Quick Placement Test (QPT), 60 intermediate L2 learners were selected at random out of 120, and then assigned to two experimental and control groups. The learners were assigned an interview of ten questions as a pretest. After that some lessons of lexical chunks using ("Collocation in Use" and "Common Idioms in English") books were thought as the treatment to the experimental group. After the instruction, an interview was conducted on both the experimental and control group as the post test. The T-test analyzed data of the post test revealed that after the treatment, the participants' fluency in the experimental group was significantly improved. The results also showed that the participants in the experimental group had positive attitudes toward explicit instruction of lexical chunks. The study has theoretical and pedagogical implications in the field of foreign/second language teaching and learning.