The objective of this study is to research the effects of using concept cartoons in the "Solar System and Beyond" unit, which is included in seventh grade science lessons, on students' critical thinking skills. The study group consisted of 58 students, selected using an appropriate sampling method, who were students in a state secondary school, which is close to the city centre. The study used a pre-test and post-test matched control group design. During the three-week-long experimental teaching process, the students in the control group (n = 30) were taught according to the curriculum of science lessons, while concept cartoons developed by the researchers were used with the students in the experimental group (n = 28). The Cornell Critical Thinking Test-Level X (CCT-X), as developed by Ennis & Millman (1989), was used as the data collection tool. The results of the study showed decreases in the critical thinking skills scores of the students in the control group, while a statistically significant difference was found in the critical thinking skills of the students in the experimental group. In addition, a statistically significant difference was observed in favour of the experimental group when comparing the CCT-X scores of the experimental group's use of concept cartoons and those of the control group's used existing teaching methods.