The mean diameter, mean basal area of individual and total basal area of stands were collected as the indices for studying the relationship between the number of trees and the size. Based on the famous -3/2 power rule, the equations were established on the relationship between density of stand and the mean diameter, mean basal area of individual and total basal area of stands in the process of the self-thinning of Pinus taiwanensis natural forest. The results showed that the population density of P. taiwanensis natural forest had significant effect on the mean diameter, mean basal area of individual and total basal area of stand. Therefore attention should be paid to control stand density in the management of P. taiwanensis natural forest.