In order to investigate the metabolic activity of microorganisms and the rate of substrate degradation during the aerobic biological treatment，a method of on-line determination of specific oxygen uptake rate（SOUR）in the sequencing batch reactor（SBR）process was established.The trends of SOUR in the organic matter degradation and ammonium oxidation processes were investigated.It was shown in the SOUR profiles that the system underwent three stages under either constant DO concentration or airflow rate conditions，i.e.deliquescent COD degradation，ammonium oxidation and endogenous respiration.The heterotrophic bacteria showed a higher DO affinity compared with the autotrophic bacteria，thereby resulting in COD removal firstly and subsequent nitrification.SOUR was 0.36 mg O2·（g MLSS）-1·h-1 during the COD degradation，and 0.18 mg O2·（g MLSS）-1·h-1 during the ammonia oxidation at the constant DO concentration of 1.0 mg·L-1.SOUR declined sharply when ammonia oxidation was completed，which indicated completion of the partial nitritation process.Aeration should be terminated to avoid further oxidation of nitrite to nitrate. Accordingly，initiation and maintenance of the partial nitritation process can be realized by identifying the slope of SOUR profile.