This study aimed to investigate the effects of livestock grazing intensities (lenient, moderate, heavy and no-grazing) on herbaceous plant species composition and diversity (grasses and forbs) in understory of Quercus spp. and Pistacia spp. forest in Kebirkouh Mountain, Darehshahr city, Ilam Province, Iran. Vegetation sampling was carried out with randomized-systematic method at flowering time of the dominant plant species in April 2017. Within each quadrat, number of species, name of plants, density, life form and palatability class were recorded for herbaceous plants. Species diversity with the Simpson’s and Shannon-Weiner indices, species richness with Margalef and Menhinick indexes, and evenness with Peet and Sheldon indices were calculated in Ecological Methodology software version 6.1.4. The highest value of Simpson index for grasses and forbs was 0.972 (moderate grazing) and 96.95 (exclosure). Also, the highest value of Shannon index for grasses and forbs was 4.51 (heavy grazing) and 4.56 (heavy grazing), respectively. The highest richness of grasses and forbs was observed in grazed area with moderate intensity and no-grazing area, respectively. In the area without grazing, the richness of forbs was 4.4 and 2.75 according to Margalef and Menhinick indexes. The values of the Margalef and Menhinick richness indexes for grasses in grazed area with a moderate intensity were18 and 15 percent higher than the grazed area with heavy intensity. Peet and Sheldon evenness indexes were the highest for grasses and forbs in heavily grazed area and the lowest for grasses in the moderate grazing intensity and for forbs in the exclosure.