Using the Weibull distribution, changes in the diameter distributions in Mizunara (Quercus crispula Blume) natural forests were analyzed. From the results, it was recognized that the Weibull parameters a, b and c increased with age. The increase of c is considered to be the most noteworthy characteristic of the diameter distributions in Mizunara natural forests. The distribution range of diameters in old Mizunara forests becomes large. Therefore, in cases where the purpose of forest management is to create a forest with homogeneous diameters, it is necessary to devise a new method of thinning.