This study examined the effects of using flipped instruction on Iranian EFL learners' speaking complexity, accuracy, and fluency (CAF). To fulfill this objective, the Oxford Quick Placement Test (OQPT) was administered to 96 Iranian EFL learners and 60 of them who were at the intermediate level were selected as the target participants of this study. Then, the selected participants were divided into two groups of experimental (n = 30) and control (n = 30). After that, the participants of both groups were given a speaking pre-test. Subsequently, the experimental group received the instruction through flipped instruction while the control group was instructed in a traditional classroom. After the treatment, a questionnaire related to the learners' attitudes towards using flipped instruction was distributed among the participants of the experimental group to investigate their ideas about using flipped instruction. The findings of Independent Samples T-test and Paired Samples T-test revealed that there was a significant difference between the post-tests of the experimental and the control groups in the favor of the experimental group. In fact, the results indicated that the experimental group significantly outperformed the control group on the post-test. Moreover, the findings of One-Sample T-Test showed that Iranian EFL learners had positive attitudes towards using flipped instruction for speaking classes. The findings of this study can encourage English teachers to use flipped instruction in their classes to reach better learning results. In addition, this study may enrich the related literature by encouraging researchers to pay more attention to the impacts of the flipped instruction on developing EFL and ESL learners' speaking skill.