Biodiversity and forest stand structure play important roles in the sustainability and dynamics of forest ecosystems, but grazing of livestocks can affect on the sustainability. In order to study the effects of grazing on forest diversity and structure, this research was carried out in Dalab region of Ilam province. Based on the number of livestock, three areas including enclosure, medium grazing and heavy grazing were identified. In each area, three transects which located on each of them ten plots, were considered. In each plot, quantitative characters and number of seedlings were measured. Indices of diversity, richness, evenness and dominance were calculated using PAST 3. 04 program. The results showed that the values of dominance and evenness indices in the enclosure condition status were less than two other conditions However, diversity and enrichment indices in livestock grazing conditions were less than the enclosure status. The study of structure shows that the grazing intensity has reduced the percentage of species in samples in comparison to the protected area. The intensity grazing effected on the parameters of DBH, trunk height, crown length, mean of crown diameter, number of branches and regeneration of Quercus brantii Lindl., Pistacia atlantica Desf., Acer monsopessulanum L., Amygdalus orientalis Duh., Crataegus pontica C. Koch, Cerasus microcarpa (C. A. Mey. ) Boiss., and Daphne mucronata Royle. Most of this parameters studied on these trees are different from the grazing situations than the enclosure status. Because intensity grazing cause restriction the growth or deployment of young seedlings in recent years or indirectly causing soil compaction to reduce fertility and permeability of the soil and cause changes in the structure of the forest. There was no significant difference between the moderate and severe grazing conditions, and in general it can be said that grazing of livestock with any intensity would disrupt forest ecosystem performance so, it is necessary to determine the grazing capacity of livestocks and prevention of overgrazing capacity in the forests.