Involving students actively in the learning process is one of the ways that can be used in developing the learning objectives of mathematics which is to improve students' critical thinking skills. Students can be called have mathematical critical thinking skills if they have a way of thinking systematically, have an awareness in thinking, and have the ability to distinguish a truth from truth, and can provide arguments from a problem. In fact, students do not have the knowledge independently before the learning process begins so the students only expect new information at school. The lack of learning resources that are easy for students to understand makes learning still go in one direction and create a passive learners. The impact is made students having difficulty in solving problems with varying level of difficulty due to insufficient time to finish it at school. These unsolved questions become independent tasks at home, and students become more difficult to solve it and caused  students have not developed the ability to think critically mathematically yet. The purpose of this study is to create a product in the form of an effective learning video to improve students' critical thinking skills in high school class XI. The development model used is Model Plomp which consists of three phases, namely the initial investigation phase, the prototype development phase, and the assessment phase. The subjects of this study were educators and students in class XI of SMAN 5 Padang. Based on the results of observations during the learning process, students look active in discussions and can solve problems that are given systematically. The results of the final test of students 'critical thinking skills, the students' mathematical critical thinking skills increased to 87.87%. So, it can be concluded that the flipped classroom-based learning device is effective to improve students' mathematical critical thinking skills in class XI.

keywords: Mathematics Learning Devices, Flipped Classroom, Critical Thinking.