Abstract: Objective To quantitatively study the effect of ages on the average apparent diffusion coefficient (ADCav) of normal-appearing white matter.Methods Fifty patients with normal-appearing white matter were divided into four age groups, namely 16-30 years, (n=13), 31-45 years (n=12), 46-60 years (n=14) and >60 years (n=11). All the subjects were examined with conventional and trace-weighted diffusion imaging in three orthogonal directions. The ADCav of the white matter was measured and compared between various age groups.Results In the white matter, the ADCav of all the patients was (0.71±0.08)×10^-3 mm²/s; the ADCav of the white matter in the first to fourth age groups were (0.69±0.06)×10^-3 mm²/s, (0.71±0.07)×10^-3 mm²/s, (0.71±0.09)×10^-3 mm²/s, and (0.73±0.10)×10^-3 mm²/s respectively. ADCav of patients between 16-30 years was significantly different from that of patients over 60 years of age (P=0.014). Patients older than 60 years had an ADCav of the white matter significantly higher than the value of (0.70±0.07)×10^-3 mm²/s in the total patients younger than 60 years (P=0.026).Conclusion Water diffusibility shows a slight increase in human white matter with aging, possibly due to mild ultrastructural changes associated with aging.