Abstract: In order to investigate the influence of nitrite accumulation during denitrification process, the nitrite accumulation in immobilized denitrification granules was studied in a lab-scale sequencing batch reactor (SBR), in which acetic sodium was used as the carbon source to control the influent ρ(C)/ ρ(N)rate. The results showed that the nitrite accumulation significantly occurred during denitrification process under different ρ(C)/ ρ(N) rate and the trend shows that it first increased, then decreased. Furthermore, the maximum nitrite accumulation occurred at 30 min when ρ(C)/ ρ(N)=4-6, then decreased slightly, and the nitrite accumulation only occurred at ρ(C)/ ρ(N) ratio 4 at the end of reaction; owing to lack of carbon source ( ρ(C)/ ρ(N)=2-3) in SBR, nitrite accumulation reached the highest level at 120min and then kept stable. This phenomenon indicated that the stable nitrite accumulation was maintained by controlling ρ(C)/ ρ(N) and the reaction time. The reduction velocity of nitrate and nitrite increased slightly followed by the ρ(C)/ ρ(N) increasing, but the accumulation rate and accumulation velocity of nitrite increased firstly and then decreased. When the ρ(C)/ ρ(N)= 4, the largest nitrite accumulation rate and velocity were 40.8% and 24.46mg/(L·h) respectively. It indicated that the nitrite accumulation was affected by the ρ(C)/ ρ(N) obviously.