The biological and ecological studies of the water resources are amongst the most important subject in research. For this purpose, identifying the ecosystem characteristics, organisms and environmental factors are the first step. Water quality assessment through permanent monitoring is the base of a program for controlling and decreasing of pollution. Therefore, They could be considered as water quality , s index in aquatic ecosystems. The use of benthic macroinvertebrates as bioandicators, is the most practical method of water quality assessment. Tajan river is one of the rivers of Caspian Sea southern sub-basins. This river originates from Hezarjarib and Alborz Mountains then traverse 120 km and drains into the Caspian Sea. Watershed area of this river is about 2000 km 2 . In order to estimate water quality indices HFBI, NSFWQI, BMWP and ASPT as well as richness and biodiversity indices of benthic macroinvertebrate, four stations were selected in Tajan river from before the city to Mahfourouz Mahalle. Bottom samples were collected monthly from May 2011 to April 2012 by Surber’s sampler (34×34cm 2 ) and in order to analysis physicochemical factors (temperature, pH, EC, turbidity, BOD5, COD, TDS, dissolved oxygen, oxygen saturation percent, nitrate, phosphate and fecal coliform), water was sampled seasonally. Overall, 9 families belonging to 7 orders and 3 phyla were identified among which the aquatic insects had the highest diversity and abundance. The results of Tajan River zoning on the base of NSFWQI showed that the quality of Tajan River is not acceptable. During the period of study, the calculated data for this index were compeletly placed it in the "poor" quality Keywords : Tajan river, Bioindicators, Diversity Indices, Benthic Macroinvertebrate, NSFWQI
