Background: The international science competitions show that students from different countries perform differently in subjects such as mathematics, physics and chemistry. In the literature comparative empirical studies tried to address the reasons for cross-national students' differences in performance from different perspectives such as teaching strategies, learning environments, curriculum and policies. However, one important area that shapes students' intellectual development is ignored in comparative study: teachers' knowledge, which eventually decides how effectively teachers teach and how well students learn. Focus of discussion: This study intends to focus on the differences between Chinese and American contexts and development of their teachers' knowledge with a greater emphasis on China. Without considering the different types of professional knowledge the teachers in these two countries possess, simple comparison of students' performance on the tests makes little sense and without considering how the teachers develop their professional knowledge, we cannot determine the primary reasons that contribute to the differences in students' performance. Suggestions: The development of teachers' knowledge is context-sensitive and a deep analysis of the context will offer a clearer picture of what teacher knowledge really is. Thus comparative studies need to look at the difference in students' performance from the perspective of educational philosophy, contexts and policies. Conclusion: If we attribute Chinese students' higher performance on tests to teacher knowledge, we first need to define the validity of the Chinese teachers' knowledge. The fact that Chinese students are excellent performers on tests does not mean they will eventually turn out to be real scientists. So far no scientists from mainland China have ever won the Nobel Prize for science. Because teachers' knowledge is either positively or negatively correlated with students' performance, to explore how teachers' knowledge has developed and under what context will reveal more meaning regarding what kind of knowledge their students have learned and a study of this kind has more practical significance for policy makers and educators. (Contains 2 notes.)