Critical thinking is one of the core twenty-first century skills. This study designs the strategy of peer assessment based on the theory of knowledge building to improve a class of 33 undergraduate students' critical thinking skills in one semester. The context of knowledge building has four phases--idea generation, idea connection, idea improvement, and rising above--which are iterated by three rounds of peer assessment. In this study, the dependent variable was critical thinking skills, and the independent variable was peer assessment. Data sources include critical thinking skills surveyed, peer comments on Knowledge Forum and grades scored by students. The results showed that (1) students' critical thinking skills were significantly improved; (2) students' reports have risen in peer assessment; and (3) students' critical thinking skills are significantly correlated with peer assessment in the knowledge-building community with both peer comments and peer grades. The findings indicated that knowledge building created a positive culture for the promotion of critical thinking skills, comments that expressed strong disagreement could improve students' critical thinking skills, and the formation of community knowledge played a substantial role in promoting students' critical thinking skills.