The aim of this study is to investigate the tree species' diversity in a region with no management planning and introduce the calculated indices as the norm for sustainability assessment. DBH of all tree individuals, and their stories were recorded in the inventory grid (75 m × 100 m) sample plots. Subsequently, richness, evenness (Simpson, Smith & Wilson) and heterogeneity (Simpson, Shannon-wiener and N2) indices were calculated. The richness index shows decreasing trend from the third story to first one. The heterogeneity indices increase from the third story to second and then decreases to first layer. These indices were calculated for the study area and also for each story, separately. Among the evenness and heterogeneity indices, Smith & Wilson and N2 showed the highest CV, respectively. Since one of the criterions in assessing forest sustainability is comparability of species' diversity in managed forest and natural woodlands, so, species' indices in the unmanaged region with natural state are proposed as a tool to evaluate forest sustainability.