In order to investigate the variation of sludge nitrification activity before and after microbulking with low dissolved oxygen(DO), with mean DO value of 0.6mg/L-0.9 mg/L，the sequence batch reactor (SBR) was employed to test the oxygen consumption velocity curves before and after microbulking. The results showed that COD removal ability increased, whereas the ammonia nitrogen removal effect was decreased with microbulking of sludge. The oxygen consumption velocity curves before and after microbulking showed that the total activity of activated sludge was 67.72 mgO2/gVSS·h before microbulking, of which nitrification activity was 43.12 mgO2/gVSS·h, that is, 63.67% of the total activity. And the total activity of activated sludge was 90.49 mgO2/gVSS·h after microbulking, of which nitrification activity was 23.98 mgO2/gVSS·h, that is, 26.51% of the total activity. The low DO value was the limited substrates for microorganism growth. When activated sludge was in the state of microbulking, the filamentous bacteria were the dominant bacteria, whereas the nitrification bacteria were subordinate. Thus the total nitrification activity of activated sludge decreased.