This research was conducted at Supra Talibura Junior High School, Sikka Regency, NTT, to determine the effect of increasing students' critical thinking skills after being given treatment using PhET Simulation in a flipped classroom setting. The trial sample used was 50 students divided into two classes. Each class was 25 people with purposive sampling. Data was collected by giving tests and questionnaires. Students are given apperception in videos a week before the learning takes place during the learning process. Giving videos aims to build students' initial learning knowledge to find out for themselves about learning topics: test t and N-gain analysis. Research results have been obtained; namely, there is an effect of using PhET from hypothesis testing and N-gain. At the same time, the results of the percentage of student response questionnaires indicate the level of student acceptance of the media used in learning.