In this study, aquatic arthropoda of Cheshmeh-Ali river in Damghan from Semnan province investigated. Collection took place from beginning of spring until Astaneh village for one year. Specimens found in rapid and calm water, algae, aquatic vegetation, muddy and sand bottoms. From insects; orders Collembula, Odonata, Coleoptera, Ephemeroptera, Hemiptera, Trichoptera, Diptera, and from Arachnida; Hydracarina, and from Crustacea; Amphipoda, Isopoda, decapoda, Cladocera and Cyclopoida collected. Cheshmeh-Ali river have relativily high stability because of it’s geographical statue. Identify 47 species of arthropoda, indicate high biodiversity in this river.