In Ethiopia one of the biggest and the most important wastewater treatment plant was located, in Addis Ababa, Kality Sub city. The treatment plant applies an activated sludge treatment method which treats a maximum of 100,000m 3 of wastewater per day. The main goal of this research was to evaluate the performance of KWWTP with the TSS, TKN, TP, BOD5, and COD parameters against Ethiopian and international environmental standards of pollution control and followed by an optimization of the plant for a better effluent quality. Laboratory analysis data were used as a main source to examine the behavior of the existing treatment plant . The result shows that the wastewater treatment plant was found as efficient in terms of removing TP, and COD concentration from the influent while, TSS, TKN, BOD 5, and NH3 concentration were beyond recommended limits. This will directly affect the quality of the environment and the aquatic life in the river, so optimization is expected to improve the treatment perfo rmance of the treatment plant. In order to optimize the treatment plant, a computer software program named GPS-X was used. The optimization of the treatment plant is addressed in two methods. The first one was to improve the effluent quality by adjusting some operational parameters like time and temperature within the existing plant, this shows a significant reduction of COD, TKN, BOD, VSS and Ammonia. Other options are considered as an upgrading the design so that, equalization tank and a sand filter were added to the treatment plant as a result a substantial reduction of COD, TKN, BOD, TSS, VSS and Ammonia.