Nowadays, critical thinking ability is one of the most important skills in the 21st century; however, not many high school students have a good critical thinking ability in English. Therefore, the aim of the research is to improve the critical thinking ability of high school students by organizing activities for students to debate in English. The specific objectives are to investigate the current situation of critical thinking ability among high school students as well as organize activities for students to debate and then evaluate the impacts of debating activities on their critical thinking ability. To conduct the study, a mixed-method research design, which combines qualitative methods and quantitative methods, was used along with the survey. There was 16 grade 10 students from CTN Debate Club at Thai Nguyen Specialized High School, Vietnam, who volunteered to take part in the research. All of these students were required to take part in speaking pre-test and post-test as well as pre and post-questionnaire about their opinions of using debating activities. The results show that using debating activities played a significant role in improving and enhancing the critical thinking ability of high school students. Students' feedback concerning the use of debating activities are, in general, also quite optimistic. Besides, the research proposes some suggestions for minimizing the drawbacks of using debating activities to enhance the potentiality of this method and make it more interesting, meaningful and effective.