Polrud river which runs in eastern part of Gilan province is about 51 km long and it flows in to the Caspian sea near Kalachay. The purpose of this research is the identification of benthos, the survey of biodiversity of the river bed ORGANISMS and its assesment potential of natural productivies. Six sampling stations were used and the work has been done on them for two years and the result of its benthos are the four class of the following aquatic insects: Ephemeroptera- Plecoptere- Diptera- Trichoptera. In winter and spring seasons, two classes of Ephemeroptera & Plecoptere nnually in upland area were dominant but Diptera &Trichoptera were permanently dominant. The average temperature of the yearly water has been 14.6°c in the first year and 14.4°C in the second year. The biomass has been identified 1.06 g in the first year and 1.11 g in the second year which most of it belongs to the Hydropsyche. The water classification of this river in the highland has been identified one and in the plain area two. The potential of natural productivity of this river has been identified 248.1 kg the first and 256.5 kg the second year in per hectare. Polrud river according to Saprobic classification lies in upland area with low Pollution and downlink area with average pollution.