This study planned to investigate the impact of utilizing formative assessment by Kahoot application on Iranian intermediate EFL learners’ vocabulary knowledge as well as their burnout level. To conduct the study, 60 homogeneous participants, who were chosen after administering the Oxford Placement Test (OPT), were randomly assigned into two equal groups as control and experimental. Then, the participants took part in the researcher-made vocabulary test as the pretest. In the experimental group, the participants used Kahoot application to address a few questions related to the vocabulary of each lesson while the participants in the control group performed the activities in their student and workbook. After ten sessions, they took part in the posttest. The analyses of the obtained data revealed that using Kahoot, as an online tool for formative assessment, had a statistically significant effect on vocabulary knowledge of Iranian EFL learners. On the other side, using this application had a statistically significant effect on reducing the burnout level of language learners. The findings could be beneficial for EFL/ESL Learners as well as teachers.