Abstarct: The Chehel- Chay stream is one the main branch of Gorgan-Rood river. The purpose of this study was to evaluate the hydrochemical water quality, concentrations of heavy mextals (Cu, Pb, Mn and Cr) and biological pollution probably occurred in the river water. To this end, 16 sampling stations were selected along the river. The results showed that, pH of the water samples ranged from 6.9-7.98, indicating neutral to alkaline in nature. Major ion concentrations show large variations (from 36.13 mg/l for Na 6.8 mg/l for NO3 respectively) in upstream area of the river where it is rather influenced by the dominant lithology mainly including limestone and however, anthropogenic sources (domestic waste and urban runoff) are affecting the river as it flows through the municipal areas. The dominant hydrochemical type of river water is calcic bicarbonate and 25% of the samples fall in hard and 75% of very hard class. Results of calculation of Saturation indexes for some reactive minerals indicates the water sample are undersaturated respect to gypsum and halite minerals and saturated relative to calcite and dolomite. Biological studies also showed that all the samples of river water (except for station S1) are polluted in terms of BOD and COD pollution. Microbial studies also revealed that all samples have degrees of microbial contamination. According to the quality indicators OWQI and NSFWQI, it is found that all samples fall in bad and very bad class. Obtained results of trace mextal analysis shows that their concentrations are below the limits set by the WHO According to Wilcox diagram, water is found to be suitable for agricultural purposes