This study investigated the Korean children’s developmental pattern of the social brain related to mentalization ability. For this study, a mentalization task and the prefrontal brain activity measurement using functional near-infrared spectroscopy (fNIRS) were administered to a total of 163 subjects in kindergarten, elementary school, and middle school. As the result, there was a significant difference in the mentalization ability score among preschoolers, lower elementary students, upper elementary students, and middle school students. However, there was no significant difference between the upper elementary and middle school students in the mentalization ability score. As a result of correlation analysis between the mentalization performance and brain activity, it was shown a significant correlation in the right hemisphere orbitofrontal cortex (OFC) and the left hemisphere dorsal lateral prefrontal cortex (DLPFC). Specifically, the correlation was analyzed as a positive correlation with the OFC and a negative correlation with the DLPFC. These results provide quantitative analysis information by taking a cross-sectional approach to the children’s developmental pattern on the social brain and mentalization ability in kindergarten, lower elementary, upper elementary, and middle school students. This study might be provide an alternative educational implication to children’s development on social brain and mentalization.