This study aimed to investigate the impact of code switching on Iranian upperintermediate EFL learners' vocabulary learning. To this end, 64 participants out of 90 male students were selected and randomly divided into two equal experimental groups- experimental and control groups. Then, the groups were pretested by a vocabulary pre-test. Then after, the participants of experimental group received the treatment, i.e., using code switching. After the treatment ended, both groups took the post-test of vocabulary. The results of paired and independent samples t-tests revealed that the experimental group outperformed the control group on the post-test. The results showed that there was a significant difference between the performance of the experimental and the control group on the post-test.