A water quality index is an essential part in the management system of water resources through to its use as a numerical scale to evaluate and classify the quality of water body for various beneficial uses (drinking, industry and irrigation). The present study used the Water Quality Index (WQI) based on Canadian Council of Ministers of the Environment, 2001 as a tool to evaluate the quality of raw and treated water for four water treatment plants in Salah Alddin province - Tikrit City form October 2010 to the end of January 2011. For this purpose, Eight variables were chosen which are: water temperature (˚C), turbidity (NTU), total dissolved solids (mg/L), pH, hardness (mg/L as CaCO₃), calcium (mg/L), magnesium (mg/L) and chloride (mg/L). The study showed that the WQI values for raw water in all plants were classified as Category III (moderate) (65.56 - 75.6). Significant improvement in treated water was noticed for consecutive months (October and December and January ). For Tikrit and Ouja water plant the WQI values are ranging between (79 – 89.77) and (79.1 – 89.73) respectively. The corresponding values for Al-Qadisiyah and Al-Qadisiyah (Al-Faris ) are (79.56 – 89.59) and (79.04 – 89.6) which means that the classification of water is within the second Category (good).