In this study water quality was indicated in terms of Water Quality Index that was determined through summarizing multiple parameters of water test results. This index offers a useful representation of the overall quality of water for public or any intended use as well as indicating pollution, water quality management and decision making. The application of Water Quality Index
(WQI) with sixteen physicochemical water quality parameters was performed to evaluate the quality of Tigris River water for drinking usage. This was done by subjecting the water samples collected from eight stations in Baghdad city during the period 2004-2010 to comprehensive physicochemical analysis. The sixteen physicochemical parameters included: Turbidity, Alkalinity (TA), Total Hardness (TH), Calcium (Ca), Magnesium (Mg), Iron (Fe), pH value, Electrical Conductivity (EC), Sulphate (SO4 -2), Chloride (Cl-), Total Solids (TS), Total Suspended Solids (TSS), Nitrite (NO2-), Nitrate (NO3-), Ammonia (NH3), and Orthophosphate (PO4-3). The average annual overall WQI was 224.32 through the study period. The high value of average annual overall WQI obtained is a result of the high concentrations of turbidity, total hardness, electrical conductivity, and total solids which can be attributed to the various human activities taking place at the river banks. From this analysis the quality of the Tigris River is classified as "very poor quality" ranging poor water at the river upstream near Al-Karhk WTP and unsuitable for drinking at the river downstream near Al-Wahda WTP and would need further treatment. The present study demonstrated the application of WQI in estimating and understanding the water quality of Tigris River. WQI appears to be promising in water quality management and a valuable tool in categorizing pollution sources in surface waters.