Some of the aquatic insects can be used as bioindicators for water pollution and as well as in a medicine and food for others animals, can play an important role in the ecosystem. The aim of this study was to investigate the biological index and aquatic insect fauna in Meshkin shahr Khiyav Chayi River in Ardabil Province. This cross-sectional study was conducted in the summer of 2020, which coincides with the season of aquatic insects.. Samples of aquatic insects were collected at 10 points of river. The biological index was determined by the formula of Hilsenhoff (1988). In general, 259 aquatic insect samples were collected belonging to Coleoptera, Ephemeroptera, Hemiptera, Diptera, Trichoptera and Plecoptera and families of Leptophlebiidae, Elmidae, Corixidae, Culicidae, Leptoceridae, Simulidae, Baulidae, Perlidae, Perlida and Hydropsychidae. In this study, the water quality index was estimated at 7.6 which in terms of quality is fairly poor in summer.