Objective To investigate agerelated changes in rat white matter with the new stereological techniques in order to figure out the exact reason for the decline of myelinated nerve fibers in aged white matter and explore the effect of sex on the agerelated change of brain white matter. Methods Nine adult Long-Evans rats (6-8 months) and 8 aged Long-Evans rats (18 months) were used. The volume density, length density, total volume and total length of unmyelinated fibers in white matter were estimated with the stereological techniques and transmission electron microscope technique. Results In the male and female rats, there was no significant decrease in the total length of unmyelinated fibers in white matter of young and aged rats. There was a significant decrease in the total volume of unmyelinated fibers in white matter of aged female rats compared with that of young female rats. The total volume of unmyelinated fibers in white matter of aged male rats was decreased by 31% compared with that of young male rats even though the difference was not statistically significant. In young and old rats, there was no significant differences of all unmyelinated fiber parameters between male and female rats. Conclusion In aged rat white matter, there was a significant decrease in the unmyelinated fibers with large diameters. Meanwhile, the myelinolysis of the myelinated fibers with small diameters in aged white matter made the age-related decrease of the unmyelinated fibers with small diameters in white matter unnoticeable.