In order to discuss the reaueing theory of simultaneous nitrification and denitrification (SND), with the sequence batch reactor (SBR), the dissolved oxygen (DO) and the floc size distribution were investigated for sewage wastewater.It is found that the SND can be attained in SBR process by nitrogen mass balance calculation with low DO (mean value 0.5 mg/L0.8 mg/L).Around 23.11% nitrogen in total nitrogen is removed by SND.The ratio of nitrate formation and ammonia decay is at 0.454 with the DO concentration at 0.5 mg/L.Meanwhile, the velocity of nitrification and denitrification is equal.And the SND couldn’t occur when the ratio of nitrate formation and ammonia decay is equal with DO at 4.296 mg/L.The mean floc size distribution of sludge is at the range of 5.02 μm ~6 μm, that is, the SND is not the results of “micro environment action” when SND occurs.