The Weibull distribution was applied to the diameter distribution of plantations, and how parameters a (min. dia.), b (scale) and c (shape) change with age or by thinning was examined. The results showed that 1) thinning generally used methods decreases parameter c, 2) parameter a was often constant, and parameter c increased up to a certain period after thinning (5-10 years), and 3) when the forests were repeatedly thinned, parameter c tended to decrease with age. These findings show that a thinning method to increase parameter c must be developed if the purpose of forest management is a forest with large c (negative skewness).