The Kan River is the largest and most important river that enters the Tehran plain. The river water quality (in Tehran city), is affected by agricultural and industrial activities and population spots. The aim of this study was to investigate the quality of the Kan River using Iran Water Quality Index for Surface Water to protect the environment. For selecting sampling stations, the study area was divided into eight longitudinal ranges, and sampling was done in two periods of minimum and maximum flow. Kan River is of poor to relatively bad quality. The pH was at standard level but some parameters such as BOD in 2 stations (minimum flow rate) and 9 stations (maximum flow rate), TSS in 4 stations (minimum flow rate) and 5 stations (maximum flow rate), DO in all samples except station S1(minimum flow rate) and coliform in all samples were more than permissible level of the national standard for river water quality. Entering industrial, municipal, and agricultural wastewater is the most important reason for the decrease in water quality. In terms of the standard classification base of the Iran Department of Environment for Protection of aquatic life, class 2 (suitable for Cyprinid (is recommended
