Considering the importance of estuaries as sensitive ecological areas and temporary habitat of many aquatic animals and also importance of Chironomidae larvae in the food chain and fish feeding, density and distribution of the chironomid larval and their relationship with environmental factors in Cheshmeh Kileh estuary of Tonekabon (South Caspian Sea basin) were investigated. Bimonthly from November 2014 to September 2015, in three stations (S₁ in the river environment, S₂ in estuary environment and S₃ in the marine environment) samples were collected using Van Veen grab (0.03 m²) and Surber (0.1 m², 0.2 mm-mesh size) with three replicates. Environmental factors of water including temperature and salinity and sediment relevant factors such as grain size and total organic matter percentage (TOM) were measured. In the present study, three subfamilies were identified, including Chironominae (5 genera), Orthocladinae (4 genera) and Tanypodinae (1 genera). 10 genera were reported from the river and estuary of cheshmeh Kileh for the first time. Among identified genera, the highest and lowest average densities were related to Orthocladius (235 ind. m^-2) and Eukiefferiella (45.5 ind. m^-2), respectively. Results of monthly distribution showed that the highest and lowest densities of chironomid larval were in March (320±133 ind. m^-2) and in September (33.3±11 ind. m^-2) respectively which were significantly different, by Duncan's test (p<0.05). Among sampling stations, river station (S₁) showed higher density (248.3±93.6 ind. m^-2) than estuary station (S₂) (90.5±29.6 ind. m^-2) and marine station (S₃) (0±0 ind. m^-2) which were significantly different (p<0.05). A significant Pearson correlation (p<0.05) between the density of chironomid larvae and environmental factors of water and sediment was found.