This study compared the pedagogical content knowledge of mathematics in U.S. and Chinese middle schools. The results of this comparative study indicated that mathematics teachers' pedagogical content knowledge in the two countries differs markedly, which has a deep impact on teaching practice. The Chinese teachers emphasized developing procedural and conceptual knowledge through reliance on traditional, more rigid practices, which have proven their value for teaching mathematics content. The United States teachers emphasized a variety of activities designed to promote creativity and inquiry in attempting to develop students' understanding of mathematical concepts. Both approaches have benefits and limitations. The practices of teachers in each country may be partially adapted to help overcome deficiencies in the other.