The distribution of  Streblospio gynobranchiata was studied. Sediment samples were collected with grab 0/1m² in spring, summer, fall and winter 2012 from 12 stations (ranging from 5, 10, 20 and 50 meters in depth) in the southern Caspian Sea from Behshahr to Ramsar (Mazandaran-Iran) coasts. Results indicated that the highest density was observed in summer in depth of 10 m (64.24 ind /0.1m²). Maximum and minimum of density was measured respectively in summer (27.22 ind /0.1m²) and winter (16.99 ind /0.1m²). A significant correlation was not observed between this species with total organic matter and silt and clay (P>0.05). For comparing the different seasons the One Way Anova was used. The results indicated  no significant differences among seasons (P>0.05).