The present study was conducted to evaluate the quality and hydrochemistry of Oqan River water using ion ratios and saturation indices. For this purpose, the results of physicochemical parameters analysis over an eight-year period (2010-2017) were used for two hydrometric stations located on the river Oqan. To analyze and identify the hydrochemical processes, Stiff, Pipers, Gibbs diagrams, ion ratios, CAI1 and CAI2 relationships and Chadha diagrams were used. Also, saturation indices and composite diagrams of saturation indices were used to predict and probability of precipitation or dissolution of some minerals. Finally, factor analysis was used to determine the number of factors affecting water quality of studied stations. Stiff and Piper diagrams results of Oqan river showed that the type and facies of hydrochemical stations are Ca-HCO3 Galikesh and Oqan stations. Based on Gibbs diagram, the water-rock reaction is the controlling factor of the water chemistry of the studied stations. The results of ion ratios and Chadha diagram indicate that reverse ion exchange and weathering of dolomite and limestone processes are two effective factors to alter water quality in the study area. Hence, river water is over saturated with dolomite, calcite and aragonite minerals, whereas it is under saturated with evaporite minerals such as halite, gypsum and anhydrite. The results of factor analysis revealed that two factors of geological formations dissolution in watershed river such as limestone and dolomitic rocks and discharge of domestic and agricultural wastewaters into river have the most influence on the water quality in the studied stations.