The emergence of flipped instruction has provided new opportunities to improve English language learning. The present study attempted to investigate the effects of flipped learning strategy on enhancing the vocabulary knowledge of Iranian EFL learners. To this end, the authors assigned 26 learners from an English institute to the flipped and conventional groups. They adopted a two-group counterbalanced design in this research. In the flipped classroom, the teacher posted the course materials via "Telegram" in advance to the class. Inside the classroom, the participants engaged in various peer and group activities including precommunicative sentence arrangement, communicative tasks, pair, and group discussion, roleplay and storytelling. The data were from multiple data sources including a vocabulary knowledge test, a student-recorded portfolio and interviews. The results revealed that the participants performed better in the conventional classroom than the flipped learning classroom. However, they did not have positive attitudes toward inverted learning. The authors presented insights into the impacts of flipped instruction on the quality of vocabulary learning and offered recommendations and implications for future practice.