The present study investigated the effect of flipped classroom on learners' critical thinking skills of inference and explanationthe flipped classroom used lecturing video as homework and group discussion as classroom activities. The participants were 60 participants aged 19-20 years old. Using quasi-experimental research, this study measured the effect size with the dependent analysis of variance (ANOVA) from the pretest-posttest score on critical thinking skills. Through 10-week intensive meetings, the two groups were identified as intact and control groups and asked to do the procedure starting from the pretest to posttest. The study was able to confirm the effect of the flipped classroom for sharpening students' critical thinking. The intervention for the EFCG was effective to help them to explore their thoughts and develop their critical thinking.