This quasi-experimental research used a two-group pretest-posttest design to compare the grammar ability between one group of students taught with an inductive approach and another group taught with a deductive approach through a flipped classroom. The study participants were Thai university students majoring in English. After the implementation, the results of the comparison between the pretest and posttest mean scores indicated a significant difference in grammar ability between the students taught with the inductive and deductive approaches in a flipped classroom. Moreover, the findings from the survey questionnaire revealed that both groups of students had positive opinions toward flipped classrooms in terms of the flexible learning environment, learning efficiency, and learning enjoyment. However, some disadvantages were also revealed such as students' lack of confidence when learning on their own, lack of motivation in self-study, lack of discipline, and inconvenience caused by Internet access. Despite some disadvantages, teaching grammar with an inductive and deductive approach in a flipped classroom can be effective to promote grammar ability of students.