Abstarct: Taleghan River is one of the important rivers in Alborz province. The study area covers a reach in Taleghan River approximately 16 km upstream of Taleghan Dam. Taleghan River initiated from the mountains Kandovan and large Kahar in north of Tehran. The aim of this study was to assess the water quality of Taleghan River. The sampling stations were selected by field inspection according to geological, land use, agricultural and residential sectors, and junctions of branches. Sampling was conducted at the end of the dry and wet seasons. Water samples were collected from 11 stations in the main river and 2 stations from fish farms. With different tests, physico-chemical properties of water samples were determined. Type and facies of water using Stiff graph and Piper diagrams, determination of water use in terms of drinking and agricultural water using Wilcox and Schuler graph, the main controller factor of the water chemistry using Gibbs model, zoning of the river water quality in GIS and water quality by NFSWQI index were the major goals achieved in this research. The results denote that in both seasons the water is in alkaline range. Most types of water in the end of dry season was sulfate-calcic and at the end of the wet season was chloride-calcic. At the end of each season, water of Taleghan River was suitable for drinking as well as agriculture use. Also, the main controlling factor in water chemistry in the area in both seasons was the weathering of rocks.