A horizontal subsurface flow reed bed, Pragmites australis, constructed wetland system was operated continuously for one year at 5 different hydraulic retention times between 5 days-11 days with real tannery industry effluent to investigate mainly NH4-N and COD removal performance of the system. The bed was also operated at different initial NH4-N concentrations between 10-30 mg/L by adding NH4Cl into the tannery effluent. The results indicated that ammonia-nitrogen removal is significantly affected by hydraulic retention time while COD is not. The optimum HRT was determined as 8 days with over 95% NH4-N and around 30% COD removal efficiency. The system shows higher removal performance at high initial NH4-N concentrations. Almost complete (99%) NH4-N removal and over 40% COD removal was obtained at (NH4-N)0 = 20 mg/L and HRT = 7 days. PO4-P and total chromium removal were not significantly affected by operating conditions