The purpose of the present study was to employ the Kahoot app which is based on game-based learning with the aim of assessing students‟ strengths and weaknesses in learning grammar through answering multiple choice grammar questions with their own mobile devices. Moreover, it made an attempt to find out the Kahoot app impact on learners‟ motivation. The study made use of the quasi experimental design. 60 female learners at the intermediate level in a private language institute in Mashhad, Iran participated in the study. They were divided to two classes. The Student Motivational Questionnaire and the Grammar Test as the pre-test, post-test were given to two-group participants. Based on grammatical point of each chapter of American English File book 3 that was already taught in both experimental and control groups, the experimental group was asked to answer 10 multiple choice grammar questions via Kahoot app as a treatment of study; while, the control group was asked to choose the right answer in their answer sheets every session. The analysis of the data was calculated by the Statistical Package of the Social Sciences (SPSS 25). The Findings of the study revealed that there was a significant effect of employing the Kahoot app on learners‟ structural ability as well as their motivation.