Abstract: The main purpose of the thesis is choosing the most superior distribution among Normal distribution, Weibull distribution, Beta distribution and Gamma distribution when they are used for forecasting and fitting Pinus kesiya var.langbianensis diameter structure of inartificial secondary forest on basis of analyzing and comparing the data from the 3 plots of Pinus kesiya var.langbianensis stands. Four diameter structure was tested by χ² test value.The result show: the most superior distribution used for forecasting and fitting Pinus kesiya var.langbianensis diameter structure of secondary forest is weibull distribution.