In order to study the polychaetes in Bahmanshir River, sampling was carried out at 5 stations using the Van Veen grab three-repeat markers at each station in February 2016 and September 2016. environmental factors including dissolved oxygen, acidity, temperature, electrical conductivity, and salinity, as well as sediment analysis including grain size and total organic matter content at each station, were measured. In this study, 6 species and 3 genera were identified from 6 families with the highest frequency belonging to the family Nereididae with 43% in the summer and 18% in the winter. The dominant species were Nereis diversicolor (Müller, 1776), Hypania invalida (Grube 1860) and Neanthes kerguelensis (McIntosh, 1885). The average total density of polychaetes was 99.99 ± 73.78 and 79.91 ± 25.59 per square meter in summer and winter, respectively. The Shannon Diversity Index ranged from 1.15 to 1.75 and the Margalf ranged from 0.39 to 0.93, indicating the poverty of the area in terms of diversity. The information obtained from this survey can determine the status of the area under study for pollution by comparing records.