Community structure and biodiversity of benthic macro fauna in around pen culture of Sturgeon fish in Gorgan Bay were studied for period of one year from August 2015 to July 2016. Seasonal samplings were carried out at 5 stations in 3 transects. In addition, depth (D), water temperature (Toc), dissolved oxygen (DO); pH, total dissolved solid (TDS), biological oxygen demand (BOD5), chemical oxygen demand (COD), phosphorus (PO4-3), nitrite (NO2-) and total ammonia (NH3+) were measured in each station. Results of season variations of Physico – chemical factors of water showed that, there were no significant differences between PO4-3 (p>0.05). However, there were significant differences between BOD5, COD, NO2-, NH3+ and TDS (p<0.05). Totally, 11 genus's and 10 families belong to 3 phyla as Mollusks, Arthropods and Annelids were identified. The results showed, there were the maximum abundance percent belong to Hydrobiidae and Cochliopidae with 33.83% and 26.25% and the minimum abundance percent belong to Gammaridae with 0.05% respectively. However, the maximum abundance belongs to Pyrgohydrobia sp., from Hydrobiidae with 3410 n/m2. The results showed, there were the maximum and minimum abundance percent in autumn and spring with 1.07 and 0.88 % respectively. The results of distance-based redundancy analysis (db.-RDA) revealed that environmental factors such as depth, DO and TDS are all important in determining the distribution of macro benthic species in Gorgan Gulf. However, there were significant differences between abundance, species number (S), diversity (as Shannon–Wiener’s, H´), species richness (as Margalef’s, D), and evenness (as Pielou’s, J) (p<0.05). There were the most diversity species in 2 and 5 stations.
