Based on observation activities in 5th grade of elementary school, the science thematic content learning outcomes are still low. Not yet achived satisfactory result is due to factors inactivity of studenta in participating in learning. For this reason, research is conducted with a type of classroom action research (CAR). This study aims to describe the steps in applying the Problem Based Learning model and to know the improvement of critical thinking skills and learning outcomes in science 5 grade science. Data collection techniques use test techniques and non-test techniques ( observation ). The analysis techniques uses descriptive comparative and qualitative descriptive techniques. The results showed that the application of the Problem Based Learning model was able to improve critical thinking skills and learning outcomes. Students' critical thinking improvement can be seen through the fairly critical category of 9% cycle, increasing in the first cycle of 36%, in the second cycle it increased to 41%. Critical category in pre-cycle 0% then in cycle I there is 18% and cycle II increases to 36%, very critical category in pre-cycle 0% then increases in cycle I to 5%, then in cycle II it increases again to 9%. While the results of learning outcomes data analysis showed that in pre-cycle the level of student completeness was 9%, for the first cycle the completeness rate was 73%, and the second cycle the completeness level was 86%. Based on the result of the study it can be concluded that the Problem Based Learning mode; can improve critical thinking skills and learning outcomes.