In this study, diameter distributions of Amance and Vallombrosa origin stands of Douglas fir were intended modelling with two parameter Weibull probability density function having shape and scale parameters for different age periods. Periodic diameter at breast height measurements evaluated in different age periods in permanent plots of planted Amance and Vallombrosa origin stands of Douglas fir in Belgrade forest Topuzlu Bend location for cultivating and investigating yield with periodic measurement of Douglas fir in Turkey by Istanbul University Faculty of Forestry Forest Yield and Economics Department were based on modelling diameter distributions. Theoretical Weibull diameter distributions were obtained with estimating the parameters of Weibull probability density function by maximum likelihood method for each origin and age periods. The suitable of observed diameter distribution of plots to theoretical Weibull diameter distribution was tested by method of Komogorov-Simirnov at 0.05 significant level. Diameter distributions obtained from plots show suitable for two parameter theoretical Weibull probability distribution, in other words it was concluded that diameter distribution of Amance and Vallombrosa origin stands of Douglas can be represented by Weibull distribution for each periods.