Critical thinking skills are one of the essential competencies that must be developed in elementary school students in the modern era. This ability serves as a foundation for students to understand problems, analyze information, and make informed decisions in subsequent educational levels. However, in practice, students' critical thinking skills are still relatively low, particularly in science learning. This study aims to investigate the effect of implementing problem-based learning (PBL) models on the critical thinking skills of 5th-grade students on human respiratory system material. The study employed a quantitative approach with a quasi-experimental method using a pretest-posttest nonequivalent control group design, which involved administering a pretest before treatment and a posttest after treatment to both the experimental and control groups. Data were analyzed using ANCOVA to determine differences in critical thinking skills after treatment. The results showed that the PBL model had a significant effect on improving students' critical thinking skills. This was evidenced by the ANCOVA analysis results, which showed an F-count of 11.414, greater than the F-table of 2.74, with a significance value of 0.001, which is less than 0.05. These findings indicate that the alternative hypothesis is accepted, and it can be concluded that the learning model has a significant effect on critical thinking skills of students on the human respiratory system material. The results of the further LSD test demonstrate that PBL has greater potential to improve students' critical thinking compared to the conventional model. This study recommends that teachers consistently apply PBL models in science and other subjects to encourage students to think deeply, systematically, and analytically