The objective of this study is to evaluate the purification level of water plant in Kosti city to supply water with high quality. This study was done to investigate the drinking water quality of Kosti city. Five sites of water network were selected, during March 2017 - May 2017. Thirty (30) samples of water from different areas of Kosti city were collected to evaluate water purification by using some laboratory tests for physical, chemical properties and microbiological. The results obtained showed that the turbidity level ranged between (4.2 – 150 NTU) thus exceeding the permissible level of WHO 1993 standards (5 NTU).The PH value was ranged between (6.3 – 8.2) and level of TDS (111 - 157 mg/L) ,hardness (43 - 89 mg/L) , residual chlorine (0.1 - 0.4 mg/L) , EC (116 - 234 μs/cm) . All these values fall below the values of national and international standards of (WHO, 1993) and Sudanese Organization for Standardization and Metrology 2008. The level of COD (86 – 584.4 mg/L), BOD (16 -144mg/L). Biological tests were conducted to determine the presence of fecal coliform the results showed that the samples of water limits according to the specifications of the Sudanese Standards and Metrology Organization (SSMO, 2008) and the World Health Organization (WHO, 1993) by 100%..