The biomass, distribution and ecological index of Polychaeta in the north coast of Gorgan Bay was assessed during spring to winter 2004. Sampling of benthic organisms was also conducted seasonally. Polychaeta species such as Hypniola annenicova (Ampharetidae) and Streblosplo gynobranchiata (Spionidae) belonging to sessil polychaetes and Nereis diversicolor (Nereidae) belonging to mobile Polychaeta were distinguished. The maximum and minimum biomass was 4489.77±712.13 and 4150~c246.37mg/m^ (2) in summer and winter respectively which showed a uniform distribution in all depths. Results indicated no difference in diversity and evenness index in depths and seasons during the year. The highest prevalence and richness of Polychaeta diversity were observed in summer especially in waters one meter deep
