Abstract: A method of treating organic waste water in an aeration tank is characterized by the fact that (1) the dissolved oxygen content in the aeration tank is adjusted to be within the range of 1.5-4.5 ppm; (2) the temperature of the waste water is controlled to be within the range of 15.degree.-40.degree. C. and the residence time of the waste water in the aeration tank is set at ten or more days if the ratio of the total carbon content to the nitrogen content of the organic materials contained in the waste water (TOC/N) is not less than 5, and the temperature of the waste water is controlled to be within the range of 20.degree.-40.degree. C. and the residence time of the waste water in the aeration tank is set at ten or more days if the TOC/N is less than 5 and not less than 2.5, and (3) the activated sludge content in the aeration tank is kept constant by controlling the amount of COD to be removed according to the equation:.sigma.T=33.5.times.1.058.sup.(T-20)wherein T is a temperature (.degree.C.