The considerable challenge of today's learning processes is to ensure that students have critical-thinking skills. This requires educators to make serious efforts to respond to this challenge. This study focuses on the improvement of the critical-thinking skill of high school students through STEM-Integrated Modeling Instruction learning. The advantage of adopting this learning model is that it provides students with extensive experience, including directly learning science through observing phenomena related to the topic discussed, asking questions after the observation, doing experiment and presentation, creating projects, and doing quizzes. This research uses a mixed-method combining quantitative and qualitative data. The qualitative data are those obtained from the result of pre and post-tests students had to take. In those tests, the researcher used the one-group design and then analyzed the data using the t-test to assess influence of learning processes on students' critical-thinking skill. Meanwhile, the later was obtained from interviews with students, in which the researcher addressed eight essay questions validated by the lecturer and revalidated empirically by 71 students. The questions were considered to be valid and reliable enough to measure students' critical-thinking skill. The results of the analyses show significant differences with alpha of 0.05 and modest improvement with N-Gain of 0.32. As a final point, STEM-Integrated Modeling Instruction learning can be considered to be applied in physics learning at schools in such a way that students' critical-thinking skill improves.