An experimental study was carried out to evaluate the performance of pilot scale constructed wetlands for the treatment of domestic wastewater in Addis Ababa, Ethiopia. Three parallel sets of constructed wetlands; consisting of one Horizontal Flow (HF), one Vertical Flow (VF) and one hybrid of HF and VFconstructed in series were built in Addis Ababa. The wetland systems had identical wetland fill media and macrophytes but with different wastewater flow types. The total surface area of the wetland systems was 72 m2 /24 m2 for each/ and designed to treat 3.15 m3 of domestic wastewater per day. Triplicate grab samples were taken from the influent and effluents every 15 days for one year and analyzed within 24 hours. Temperature, pH, DO and EC were measured onsite and the nutrient content of macrophytes was determined twice during the monitoring period. During the first 12 months monitoring period, the average removal efficiencies of the HFCW, VFCW and hybrid CW were: BOD (89.1%, 92.2% and 93.4%), COD (80.6%, 82.1% and 84.0%), TSS (89.1, 83.8% and 84.7%), NH4 + (58.6%, 66.2% and 65.4%), NO3 - (64.0%, 71.5% and 73.5%), TN (49.1%, 54.9% and 58.7%), PO4 3- (45.4%, 50.3% and 48.4%), TP (58.0%, 51.7% and 54.4%) and FC (98.6%, 96.6% and 96.5%), respectively. The hybrid system showed relatively higher removal efficiencies for most pollutants. Again, the wetland systems showed relatively higher percent reduction during the dry seasons /from Dec - May/. The areal removal rate constants of BOD5, TN, PO4 3- and TP were higher than the literature values while the values of COD and TSS were lower compared to the literature values. Concerning the nutrient content of the wetland plant, the average TN contents of the below-ground and above-ground plant part were 1.56% and 2.27% for the HFCW, 1.75% and 2.74% for the VFCW and 1.80% and 2.63% for the hybrid system, respectively. Meanwhile, the average TP contents of the belowground and above-ground plant part were 0.139% and 0.064% for the HFCW, 0.167% and 0.067% for the VFCW and 0.115% and 0.065% for the hybrid systems, respectively. In general, the results showed that properly designed constructed wetland systems could be used as effective wastewater treatment method in Ethiopia. / Environmental Sciences / Ph. D. (Environmental Sciences)