Bole Lemi industrial park is one of the industrial parks located in Addis Ababa near to Goro with an area of 155 hectares. It has 20 manufacturing sheds. 18 sheds produce garments like jeans, gloves and underwear. The rest produces shoes. Those factories produce jeans clothes are four in number and has laundries. The laundries discharge 1500m 3 /day of wastewater. So to treat the wastewater, it has common effluent treatment plant (CETP). The aim of this research was to evaluate the performance of the CETP and optimize the treatment plant for better effluent quality. Laboratory analysis was used as a main source to examine the behaviour of the existing treat ment plant. Then effluent quality generated from the CETP was assessed to inspect the impact after disposal. The performance evaluation of the CETP and data analysis was done using SPSS and Microsoft Excel. The optimization of the CETP was performed using Hydromantis GPS-X 7.0 Software to improve the quality of the effluent discharged. The laboratory results showed that COD, BOD5, TKN and NH3-N values were within the range according to Environmental Standards for Industrial Pollution Control in Ethiopia. But, pH and TSS were not within the range. Therefore, the CETP is performing well except pH and TSS. The efficiency of the treatment plant was around 57.7% for TSS, 65.2% for VSS, 81.2% for COD, 87.4% for BOD5, 38.3% for TKN and 14.7% for NH3-N. The optimization of the treatment plant was addressed on the improvement of the effluent quality. The optimization was done using GPS -X 7.0 software. It was done without dosing lime. Then, t he TSS was improved from 98.2mg/l to 18.98mg/l, the COD changed fr om 78.6 to 62.76, BOD5 from 25mg/l to 23.69mg/l and TKN improved from 20mgN/l to 18.51mgN/l. So, it gave a big improvement on TSS, COD, BOD5 and TKN.