The purpose of this study was to examine the contents of pedagogical content knowledge of an elementary school experienced expert mathematics teacher, its development process, and the factors that affect the development of pedagogical content knowledge. This study adopted qualitative research methods in the collection of data, which included naturalistic observations, interviews, and documentary analysis. After six months of exploration, the following are several imperative discoveries to this study: 1. The contents of case teacher's pedagogical content knowledge consists of knowledge of: educational goals, learners, curriculum, contexts, and general pedagogy. 2. Initial pedagogical content knowledge is introspected from early life experiences of the case teachers and from formal educational experiences strengthened through practical instruction at the center for teacher education. 3. The formation of pedagogical content knowledge is greatly influenced by the case teacher's pre-working education, which had gradually developed into strategical knowledge after the evidence of practical instruction. 4. The vast pedagogical content knowledge of this case teacher had generally developed from practical teaching. 5. The personal characteristic traits, the earlier educational experiences, the educational curriculum in the center for teacher education, the continuing training at the mathematical experimental class, the practical teaching experiences, and collegial influences all affect the development of case teacher's pedagogical content knowledge.