The aim of this research was study of tree species diversity in the Zagros forests of Iran that known as Ardal’s Chartagh protected forests. The total of the study area was 50 hectares. In this way 50 sample plots (circular shape with 1000m²) were measured based on systematic-random sampling method. In each plot species, DBH, species and number of trees were measured and recorded. To estimate of the tree species diversity, Shannon-Wiener, Simpson, Mc Arthur’s N₁ and Hill’s N₂ indices and for calculation of evenness and species richness Wilson- Smith’s and Margalef indices were used respectively. Also species importance value (SIV) was calculated. The results showed that there are 7 tree species in the study area including Fraxinus rotundifolia Mill., Juniperus polycarpos C. Koch, Cerasus mahaleb (L.) Mill., Quercus brantii var. persica, Pistacia mutica Fischer, Acer monspessulanum subsp. cinerascens and Salix persica Boiss.. Values of Shannon-Wiener, Simpson, Mc Arthur’s N₁ and Hill’s N₂ indices are 1.23, 0.58, 2.40 and 2.16. Also Wilson- Smith’s and Margalef indices showed that the values of richness and evenness are about 0.84 and 0.77. The results of SIV showed that the highest value of SIV belonged to the Fraxinus rotundifolia Mill. (188.63%) and the lowest value was due to Salix persica Boiss (17.59%). Therefore, conservation of forest stands might be suggested as a suitable approach for increasing plant diversity.