Different diffusivity measurements in diffusion-tensor imaging (DTI) could be helpful for detecting the distinct mechanisms of white matter degeneration in Alzheimer's disease (AD). However, few studies have explored the changes of white matter in amnestic mild cognitive impairment (aMCI) and AD by whole-brain voxel-wise analyses of all diffusivity indices. The association between grey matter atrophy and white matter damage measured by distinct diffusivities is still uncertain. Structural magnetic resonance imaging and DTI with four diffusivity indices, comprising fractional anisotropy, mean diffusivity, axial diffusivity, and radial diffusivity, were performed in 30 normal controls, 26 mild AD patients, and 40 aMCI patients with isolated memory impairment. T1 voxel-based morphometry and DTI tract-based spatial statistics were applied to compare the grey and white matter changes in the 3 groups. In contrast to the lack of significant white matter change presenting in aMCI patients, extended white matter degeneration over entire cerebral networks was exhibited in mild AD patients. Both axonal degradation and demyelination contributed to the white matter degeneration in AD; nevertheless, demyelination essentially involved the frontal portion of cerebral networks. Axonal degradation and demyelination over the temporal region were associated with the contiguous grey matter atrophy. However, only the severity of demyelination over the frontal region was correlated with the degree of atrophy over adjacent frontal grey matter. Our results suggest that different mechanisms of white matter damage demonstrate discrete regional distribution in AD. Demyelination may independently correlate with contiguous grey matter over the frontal region.