Objective To explore the changes of the white matter and the myelinated fibers in the white matter of aged rat brain. Methods The white matter and the myelinated fibers in the white matter of 10 young Long\|Evans rats (5 female and 5 male rats, 6\|8-month-old) and 9 aged Long\|Evans rats (5 female and 4 male rats, 18 month old) were quantitatively investigated with stereological techniques and transmission electronic microscopy. Results When compared with those of young rats, the total volume of white matter and the total length of myelinated fibers in the white matter of aged rats were decreased by 24.1% and 41%, respectively. In the white matter of aged rats, the volume density of myelinated fibers, the volume density of myelin sheathes and the mean diameter of myelinated fibers were increased by 30%, 23.9% and 31%, respectively, when compared with those of young rats. However, there were no statistically significant changes of the length density and the total volume of myelinated fibers and the total volume of myelin sheathes in the white matter of aged rats when compared with those of young rats.Conclusion The atrophy of normal aged brain is mainly due to the reduction of the white matter volume. The total length of myelinated fibers in the white matter of aged rats is signigicantly decreased when compared with young rats, which is primarily caused by the loss of myelinated nerve fibers with thinner diameters in the white matter.