White matter (WM) is critical to cognitive function and brain activity. The objective of the present study was to test whether diffusion tensor imaging (DTI) derived WM measures are related to the cognitive event-related potential error-related negativity (ERN). Eighty-seven healthy middle-aged adults underwent DTI scanning and electrophysiological recordings while doing a version of the Eriksen flanker task. ERN was elicited in error trials. Fractional anisotropy (FA) was calculated based on the DTI scans. FA indexes degree of anisotropic diffusion in every voxel, and is assumed related to the integrity of myelinated fiber bundles. The principal neuronal generator for ERN is located in the anterior cingulate cortex (ACC). Hence, the relationship between FA in the cingulum bundle and ERN amplitude was tested. It was found that FA in the left posterior cingulate correlated with ERN. Eigenvalue analyses revealed that radial diffusivity was responsible for the FA effect. ERN amplitude predicted response accuracy in the Flanker task, suggesting that electrophysiological measures are intermediate explanatory variables connecting DTI indices of WM organization, synchronization of large cell assemblies, and behavior.