Trees are the most important biological elements of forest ecosystems. The variability of the tree species composition inhabiting in the Oriental beech forest, not only forms different forest types but also has a remarkable impact on the species diversity of forest floor plants, due to the existence of trees in the overstory layer. In this research, forest types of an an Oriental beech were identified and their impacts on the species diversity of forest floor plants was compared and evaluated. Tree inventory was done at 16 hectares (100%) for two layers of trees and forest floor plants. The forest types were determined using the basal surface area of tree species and by the table synthesis method. The species diversity were calculated using richness index, Shannon-Wiener indices, Hill numbers (N₁ and N₂) and modified Hill (E₅), Simpson. Evenness were calculated using Simpson, Camargo, Smith and Wilson indices and modified Nee. Analysis of variance showed that the impacts of forest types on plant diversity indices were significant. Differences between forest types were not significant in terms of evenness indices. Comparing the mean of tree species diversity indices in different forest types showed that Beech-alder and Beech-maple types had similar values. Whereas in the forest floor vegetation layer, diversity indices were highest in Beech-maple type and lowest in Beech-alder type. Maple and Alder have different ecological characteristics, thus changed the forest environment in different ways, resulting variability in species diversity of forest floor plants. These findings can be applied for comparing the forest developmental stages and also determining and conserving the diversity hotspots.