Cognitive impairments and white matter (WM) microstructural alterations have been found in subjects with bipolar disorder (BD). However, the relationship between WM microstructural alterations and impulsivity, a prominent cognitive trait, in children/adolescents with BD is not known. In this study, diffusion MRI and cognitive assessments were obtained from 19 children/adolescents diagnosed with BD and 23 age-matched healthy controls. We found increased radial diffusivity(RD), reflecting disrupted myelin, in major WM tracts such as corpus callosum. Significant correlation between RD in WM tracts regulating impulsivity and response time to affective words was found, suggesting the association between WM myelin disruption and impulsivity.