The aim of this study was to develop and test a new basal area growth model in mixed species continuous cover forests in north Iran.We analyzed 421 core samples from 6 main species in the forest area to develop our growth model.In each plot,we measured variables such as total tree height (m),diameter at breast height (DBH) (cm) and basal area of larger trees as cumulative basal areas of trees (GCUM) ofDBH ＞ 5 cm.The empirical data were analyzed using regression analysis.There was a statistically significant nonlinear function between the annual basal area increment,as the dependent variable,and the basal area of the individual trees and competition as explanatory variables.Reference area from the largest trees,was circular plot with area of 0.1 ha.GCUM was estimated for trees of DBH ＞ 5 cm.Furthermore,we investigated the dependencies of diameter growth of different species on stand density at different levels of competition,and diameter development of individual trees through time.The results indicate that competition caused by larger neighborhood trees has a negative effect on growth.In addition,the maximum diameter increment is affected by competition level.Therefore,the maximum diameter increment of species occurs when the trees are about 35-40 cm in dense-forest (40 to 0 m2 per ha)and when the trees are about 60 to 70 cm in very dense forest (60 to 0 m2 per ha) which is more likely to Caspian natural forests with high level density due to uneven-aged composition of stands.