Water treatment is any process that improves the quality of water to make it more acceptable for a specific end-use. Safe water is used the greatest challenge facing the world today. The key problems of the water treatment plant were, filtering deteriorated water to the community. This study was conducted to evaluate drinking water treatment plants in the Gambella City from January to end of July 2018 G.C because of suitability for seasonal variation of dry and rainy season. Primary data was collected from laboratory analysis and observation. Different method was used to evaluation the efficiency of treatment plants including water quality characterization and calculating removal efficiency by comparing with world health organization and EPA standards. Four representative sample sites were selected during data collection including raw water, the sedimentation basin, the filtration basin and the final stages after the chlorination. The result shows that, in dry season the value of Turbidity color, E.coli was 11mg/L>5 mg/L, 40.83>15 Mg/l Pt., 1 > 0 CFU/100ml respectively. Whereas during rainy season Turbidity, color, Aluminum, E.coli was 30mg/L, 113>15, 0.42>0.2 and 2>0 respectively after treatment. All those values were above World health organization standards and the overall efficiency of treatment plants was 58% and 48% for dry and rainy season respectively. From this result, it can be conclude that the treatment plant was not efficient to remove pollutants such as turbidity, aluminum, E.coli and color.