Flipped Learning is a rather new approach to teaching. In this approach, the place of teacher's lectures in the classroom and homework assignments are exchanged to enhance active learning, engagement, and motivation. The aim of this research was to determine the effect of the flipped classroom model on intermediate and upper-intermediate learners' engagement and motivation in learning English grammar. In the current study, quasi-experimental research was used. This research was conducted within the English course for six weeks' period in the summer term in 2019. Three hundred and sixty English learners have been chosen through multi-stage cluster sampling from four different institutes. They were randomly assigned to control and experimental groups. Data were collected both pre-test and post-test. Among 50 grammar videos, eight grammar contents based on CVI (content validity index) CVR (content validity ratio) (0.42, 0.79) were selected. Questionnaires' validity was checked by some experts and their reliability was checked through Cronbach's Alpha test. Questionnaires were Hermense (motivation, ? = 0.97) and SEI (engagement, ? = 0.93). In the current study, descriptive statistics, and ANCOVA tests were used in the analysis of the quantitative data. Results indicated that learners in the flipped classroom performed better than learners in the traditional class. It was concluded that there was a significant difference between the pre-test and post-test scores of the experimental group whereas there was no significant difference between the pre-test and post-test scores of the control group. The findings of this study carry important implications for foreign language syllabus designers, language instructors, etc.