I n this research, used convential activated sludge reactor using spray aerator as an aeration source. As raw material , used synthetically wastewater which has characteristic as same as domestic wastewater. The purpose was for continuity in getting kind of concentration that would be used, This synthetically wastewater was made using BOD:N:P = 100:5:1. This ratio was chosen to make sure that the nutrients were enough in high rate biological treatment. This researchwas divioded into 2 parts. Those were pre research and primary research. In pre research, included seeding and acclimatization. Seeding was aimed to grow microorganism would be used until microorganism concentration was enough to degrade organic compund in synthetically wastewater. This seeding was happened for 29 days with MLVSS on the last day was 2890 mg/l. After seeding foinished, than acclimatization was done. Acclimatization was aimed to adapt microorganism with one of operational condotions that would be applied. Those were organic compound concntration (BOD5) 400 mg/l. aeration time 4 hours, sludge age 10 days and resirculation 50 %. This acclimatization was happened for 10 days with efficiency of degrading organic compund 88.836 %. Than, primary research which in this research was given variation of organic compound concentration (BOD5) of 400 mg/l, 500 mg/l, 600 mg/l and 700 mg/l into conventional activated sludge reactor. In every variation , measured some parameters that were permanganate value. MLSS, MLVSS, DO temperature and pH. Those were aimed to know effects of those paramneters in degrading compound concentrations. Operational conditions applied in this research were aeration time 4 hours, resirculation 50 % and sludge age 10 days. In BODs concentration of 400 mg/l, was getting efficiency 92, 79 %. In BODs concentration of 500 mg/l, was getting efficiency 94, 31%. In BOD s concentration of 600 mg/l, was getting efficiency 84.14 %. And in BODs concentration of 700 mg/l, was getting efficiency 94.64 %. From this research, the best efficiency was 94.34 % for BODs concentration of 700 mg/l with F/M ratio 1924-gr. BODs/gr day and organic loading 4.201-kg BODs/m3 day