Abstract: The dynamics of the diameter structure of Pinus kesiya var. langbianensis natural stands and the density effect of the stands were studied with the criteria including skewness, kurtosis, diameter variance coefficient, cumulative diameter distribution curves and frequency distribution based on the data from five periodical forest resource survey on P. kesiya var. langbianensis in Yunnan Province．The results showed that in the 10-70a age classes, the skew value of the stands changed gradually from positive to negative, the absolute value of the skew index changed from small to bigger, and the stands with high density had bigger skew value. The value of kurtosis showed the downtrend with the increase of stand age. The kurtosis value of the stand diameter distribution increased as the stand density increased. No matter how the stand density changed, the value of kurtosis all tended to 0 at last. The study showed that the diameter variance coefficient generally decreased along with the increment of stand age, and it was not obviously influenced by stand density. At any range of cumulative frequency distribution, the smaller the density was, the bigger the middle value of diameter class was. The sizeclass distribution directly testified these conclusions.