The water quality index (WQI) is a very effective method that allows to compare the quality of various water samples based on the indicator values of each sample. In this study, water quality index for Basrah water supply was determined by choosing nineteen water treatment plant (WTP) in Basrah city. Twelve chemical and physical parameters of each WTP were analyzed for one year during 2011. The results show that the WQI values of water supply in Basrah city are ranged from 83 to 275. About 10% of water supply can be classified as a good water, 74% can be classified as a poor water and the remaining 16% are very poor water. The prime cause of deterioration in Basrah water quality is the poor quality of the raw source water represented by Shatt al-Arab river, due to the large amount of contaminants are discharged in it. In addition, it is affected by the tide phenomenon of Arab Gulf which causes increase of salts concentrations. Also, the WTPs in Basrah city are conventional type that do not deal with soluble elements. These plants need upgrading by adding filter membranes or ion exchange units, to produce safe water for human consumption.