Professional development (PD) has been widely implemented to shift mathematics instruction practices from teacher-centered (traditional practice) to studentcentered. As such, a study examining mediating factors on the relationship between PD and student-centered instruction is of great importance in enhancing the effectiveness of PD and shifting teacher practice. Although studies examining the mediating effects of contextual factors (e.g., administrative support) on the relationship between PD and instruction abound, studies exploring the effects of teachers' psychological factors on this relationship are relatively scarce. In this study, we examined the mediating effects of mathematics teachers' self-efficacy beliefs and classroom stress on the relationship between PD and student-centered instruction. We used a secondary data set collected for the 2015 Trends in International Mathematics and Science Study (TIMSS). For this data set, 317 teachers of eighth-grade mathematics, selected from 150 schools in Korea, were surveyed, and an analysis was conducted using structural equation modeling. Our findings revealed significant mediating effects of self-efficacy beliefs on the relationship between PD and student-centered instruction. However, the mediating effects of classroom stress were not significant. Moreover, we found significantly positive direct effects of PD and self-efficacy on student-centered instruction.