The ability to think critically in the 21st century is a skill that must be possessed by every individual. This study, uses a UbD-based problem-based learning model with a conventional learning model to see which learning model has more influence on the critical thinking ability of grade IV students in science subjects. The UbD approach was chosen to be a supporter in the implementation of the problem-based learning model because it is considered to have the same goal, which is to provide students with a deep understanding so as to encourage students to be more active in the learning process which also encourages students to think critically. The research method used in this study is quasi-experimental. Researchers used all students in class IV A totaling 25 students as the control class and IV B totaling 32 students as the experimental class. The experimental group was given UbD-based problem-based learning model treatment while the control class was given conventional model treatment. The results showed that the experimental class with UbD-based problem-based learning model treatment was superior to its post-test value compared to the control class. The findings of this study can help teachers or instructors to find a suitable learning model that has a deep impact on students, so that students can improve their critical thinking skills, which is a must-have ability in the 21st century.