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The main objective of this study was to estimate the effect of decline disturbance on diameter distribution of coppice oak trees in central Zagros region. For this purpose, two similar stands with equal areas (32ha) in central Zagros (Shorab in Lorestan province and Zarabin in Ilam province) were selected. Diameter at the breast height of declined trees was recorded before and after cutting using 100% inventory method. Following probability density functions (PDFs) Beta, Jonson, Weibull, Gamma, Normal and Log-normal were fitted to the data. The results of goodness of fit tests shown that Beta, Weibull and Jonson PDFs were fitted well to diameter of woody stems in Shorab but Jonson, Beta PDFs were much m ore suitable for Zarabin region. The difference of diameter at hight breast was studied in two stages of before and after declining by means of mortality PDF and using different PDFs. There was a big difference in diameter classes between before and after decline disturbance according to mortality distribution. There was no difference between before and after decline disturbance based upon statistical distribution fitting criteria. Outbreak of decline within this region had merely similar influences on all diameter classes, and as a result, it had not a significant effect on overall diameter fit.