Differences in tree species diversity of natural mixed stands were compared between the eastern Black sea region (EBSR) and western Mediterranean region (WMR) of Turkey, to clarify the effects of differences in forest structure, focusing on the tree species occurring in each. Species diversity, with special reference to stand structure, of natural mixed stands was quantified by Shannon-Wiener index (H'), equitability index (J'), and species richness index (R). All species diversity indices were significantly higher in EBSR than in WMR. According to the coefficients of homogeneity (CH), stands in EBSR are generally have uneven-aged stand structure, but in WMR even-aged stand structure is more common. Uneven-aged stands have more tree species diversity than even-aged forests due to complex vertical forest structure and species composition. According to Pearson's coefficients, species diversity indices and richness are closely related the average stand diameter (Ds) in uneven-aged stands of EBSR, but average-stand diameter (Ds), age(A), and stand density (SDI) are the most important stand parameters in even-aged stands of WMR in Turkey.