Abstract: This study presents a new characterization on tree diameter distribution problem. The main purpose of this study is to investigate the relationship between the stochastic Gompertz shape diameter growth model and diameter distribution law using stochastic differential equation methodology. The probabilistic characteristics of diameter growth model, such as the univariate transition probability density of tree diameter, the mean and variance of tree diameter is established. A generalized form of mean volume, volume increment and diameter increment of a tree is introduced which implicitly incorporates an age-height-dependent transition probability density function of tree diameter. To model the tree diameter distribution, as an illustrative experience, is used a real data set from repeated measurements on permanent sample plots of pine stands in Dubrava district at Lithuania. The results are implemented in the symbolic computational language MAPLE 11.