Abstract: Following the quantification theory I,a new non-linear regression model is established as follows:where Y for tree height;t,tree age and x, site factor. In the model parameters could be calculated with the improved Gauss-Newton iteration method. This statistical model could not only be utilized to evaluate site quality but also precisely to predict height growth and site productivity for the different aged stands. The statistica; accuracy could be obviously increased using this model in multiple correlation ratios were 0.9298 and 0.7904, residuals 1.0955 and 1.9636, respectively.

Key words: Quantification theory, Non-linear regression, Iteration, LS-estimation