Skill in the 21st century that must be mastered by students is critical thinking skill. Students’ critical thinking skill can be developed through the world of education. The results of observations found at Bireuen State Islamic Senior High School shows that the students at Bireuen State Islamic Senior High School have low critical thinking skill because the learning conditions tend to be passive, so they cannot explore critical ideas in solving the problems presented. These conditions cause students to only refer to the answers provided by the teaching concept, namely conventional modules to answer the problems given, without observing the problem directly. The research objective is to find out the improvement of students’ critical skill through the use of free inquiry-based modules. A quasi-experimental with pre-test and post-test control group design was used in this study. The population of the study was Islamic Senior High School 4 Bireuen and Islamic Senior High School 5 Bireuen students, totalling 165. The sampling technique used random sampling, so the sample in this study amounted to 111 students. The instrument used to measure critical thinking skill is a reasoned multiple-choice test. The data were analyzed by using N gain and analyzed by using the independent sample test at a significant level of 0.05. The result of the study is p < a, which is 0.001 < 0.05. The application of the free inquiry-based module can improve students’ critical thinking skill at Islamic Senior High School Bireuen. The N gain value shows 0.78, an increase in the high category. There are differences in the critical thinking skills of students in the experimental class and the control class on the concept of environmental pollution at Bireuen State Islamic Senior High School. The implication of this study is that this module can be used as an alternative teaching concept that refers to the free inquiry model, which will facilitate educators in carrying out the learning process in the classroom and guiding students in constructing their knowledge. From the results of this study, it is recommended for other researchers to increase creativity and innovation in learning such as by using free inquiry-based modules to support classroom learning so that students' critical thinking skill can be improved.

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