The removal of organics and nitrogen was investigated in a sewage treatment plant by oxidation ditch process. Influent and effluent samples were taken each month for the 2-year period and examined. The wastewater with average BOD value of 240 mg/l was treated in an oxidation ditch at a periodical average MLSS concentration of 3,100 mg/l. Average BOD of the effluent was 19 mg/l and BOD removal averaged 94%. Average total-N in the effluent was 11 mg/l, the bulk of which was composed of Kjeldhal-N, and nitrogen removal averaged 74%. Alkalinity reduction was about 3 mgCaCO3/l per apparent decrease of 1 mg/l ammonia-N, though the theoretical value by nitrification is 7.14 mgCaCO3/l, and no large pH drop was observed in the effluent. Alkalinity loss by nitrification was considered to be mitigated by generation of alkalinity by deamination and denitrification. Apparent 50% ammonia-N removal corresponded to the residual alkalinity of 110 mgCaCO3/l.