The purpose of this study was to examine the effects of critical thinking in International Baccalaureate (IB) classes in Japan. This study helped to determine the effects of using critical thinking methods in IB classes to determine to what extent critical thinking methods affect students' critical thinking skills, Japanese language skills, interest in social issues, and social attitudes. This study also served to investigate the relationship between critical thinking skills and critical dispositions. This study took place in a private high school in Japan. The research sample consisted of 203 11th-grade students who participated in a student response system over the course of 12 weeks of the first semester of 2018. There were 15 students (2 males, 13 females) in an IB class, 77 students (39 males, 38 females) in non-IB arts-related classes, and 111 students (54 males, 57 females) in non-IB science-related classes. The IB class emphasizes critical thinking in classroom teaching methods and activities, while the non-IB classes do not especially utilize critical thinking in the classroom. In order to examine the effectiveness of critical thinking in class, experimental research was used. Critical thinking skills tests, critical thinking dispositions surveys, and Japanese skills tests for both a pretest and posttest were administered. According to the results, there was not a significant correlation between critical thinking skills and critical thinking dispositions of students enrolled in the IB class or students enrolled in the non-IB classes. However, there was a significant correlation between critical thinking skills and Japanese skills of students enrolled in the non-IB classes. The difference of the females' mean score between the IB class and the non-IB arts-related and non-IB science-related classes was significant. The critical thinking dispositions mean score of students enrolled in the IB class was significantly higher than the mean score of students enrolled in the non-IB arts-related and non-IB science-related classes. In addition, the Japanese skills test mean score of students enrolled in the IB class was significantly higher than the mean score of students enrolled in the non-IB arts-related and non-IB science-related classes. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com.bibliotheek.ehb.be/en-US/products/dissertations/individuals.shtml.]