In order to investigate variations in plant diversity in Dr. Dorostkar's forest reservoir, located in Gisoum Talesh, 44 sampling plots were applied by the randomized-systematic method. The diameter of all tree species (dbh?5 cm) was measured in 400 m2 (20×20 m) square sampling plots. In addition, the number of individuals of shrubs and saplings (DBH?5 cm) were enumerated in circular 100 m2 sampling plots. The cover percentage of the herbaceous species was identified by minimal area method using the Domin criterion. The indices of biodiversity, including Shannon-Wiener (H?), Simpson (1-D), Hill’s N1 and Hill’s N2, indices of evenness, including Shannon-Wiener and Simpson and richness indices, including Margalef and Menhinick were applied on tree, shrub, regeneration and herbaceous layers separately. The Species Important Value (SIV) of vegetation cover indicated that tree, herbaceous and regeneration species had lognormal distribution and shrubs species had geometric distribution in this area. Results showed plant diversity were low, due to Box tree (Buxus hyrcana Pojark.) which is shade tolerant. The diversity and richness were highest in herbaceous layer and decreased from herbal to tree, regeneration and shrub species, respectively. The highest evenness was found in tree layer.