This study aimed to investigate the extent to which L2 learners' use of L1 improved their speaking ability. Employing a quasi-experimental research design, the researchers studied the effect of learner code-switching on their speaking accuracy, fluency, and complexity scores. The participants of this study were 117 lower-intermediate students receiving English instruction in eight intact classes in different branches of a private institute. These classes were assigned to two +code-switching and –code-switching treatments randomly. While the participants in the +code-switching group could use Farsi for at most 30 seconds in each activity, those in the –code-switching group were not allowed to switch to their mother tongue. The findings of this study showed that there were no significant differences between the students’ speaking ability in terms of fluency and accuracy across the two groups, the complexity scores of +code-switching group, however, was significantly higher than those in –code-switching group. The results also showed the significant speaking improvement of the students in both groups. The findings of this study suggest that the use of controlled amount of learner code-switching could be useful when complexity is the focus of an activity. Furthermore, no sign of detrimental effect of learner code-switching was traced in our study.