Between ages 4 and 6, children become better at inferring what others are thinking and feeling. However, the neural correlates of these advances are understudied. The current study investigated the relation between performance on a face-based mental state inference task and white matter characteristics. Two tracts of interest, the uncinate fasciculus (UF) and inferior longitudinal fasciculus, were analyzed due to their involvement in social-emotional and face processing, respectively. Findings demonstrate a significant relation between fractional anisotropy in the UF and task performance in 4- but not 6-year-old children. Findings have implications for typical and atypical populations.