In this study the research was focused on the mechanisms of how Chinese in-service teachers' beliefs about mathematics, together with their professional knowledge, affect their teaching approaches. With a quantitative survey study involving 92 in-service teachers, it identified six types of teachers with three different categories of beliefs (i.e., Instrumentalist, Platonist, and Problem-solving oriented) and two different levels (i.e., strong and limited) of subject knowledge and pedagogical content knowledge on the topic of functions. Six teachers were subsequently selected, for a detailed analysis of how the teachers' beliefs and knowledge influenced their teaching practices. The case study results showed that teachers with different professional knowledge levels about functions and beliefs about mathematics had different teaching focuses and showed different teaching approaches. Three teaching approaches were identified, including the performance-focused content-centered approach, the understanding-focused content-centered approach, and the thinking-focused student-centered approach.