The Zagros forests are important and valuable forest ecosystems in Iran. In order to study plant species diversity and its relationship with environmental factors in the central Zagros forest ecosystems, 600 hectares of forest of the western Dena protected area were selected near Yasuj City. 50 plots of 450 square meters were then established with dimensions of 15× 30 meters. So, 200 microplots of one square meter, 200 microplots of 10 square meters and 50 microplots of 45 square meters were established within the main plots. All physiographic factors, such as slope, aspect, altitude, and the physical and chemical soil properties of each plot, were collected and measured. All tree and grass species on each plot were recorded. The results showed that factors such as lime, potassium and soil texture had diverse effects on trees and grasses. Thus, by increasing lime and decreasing potassium and larger soil particles, grass biodiversity increased, while tree diversity decreased. Altitude and aspect had an affect on tree richness, and slope affected grass diversity. Lower altitudes and northern aspects showed a higher amount of richness, and the middle slope had the highest value of grass diversity. Also, the effects of soil factors on grass biodiversity were more than physiographic factors. The present study also determined that biodiversity, especially grass biodiversity, despite protection, was low in the available areas and lower slopes, due to human harvests and livestock grazing.