Characteristics　and　mechanisms　of　nitrite　accumulation　during　wastewater　denitrification　were　reviewed　based　on　our　experimental　results　and　literatures　reports.　Nitrite　accumulation　may　be　caused　by　the　rate　difference　between　nitrate　reduction　and　nitrite　reduction.　It　can　also　is　the　results　of　some　species　of　bacteria　or　enzymes　absence　in　the　denitrification　sludge.　Although　literature　reports　were　inconsistency,　temperature,　dissolved　oxygen,　pH,　types　of　carbon　source　and　C/N　ratios　are　the　main　environmental　factors　that　have　a　big　influence　on　nitrite　accumulation.　Nitrite　is　detrimental　to　aquatic　organism　if　the　wastewater　containing　enough　nitrite　was　discharged　to　waters　or　can　be　a　big　problem　for　activated　sludge　process　operation.　Nitrite　accumulated　in　denitrification　may　serve　as　electron　donor　for　anammox　reaction　and　the　demonstration　tests　are　undergoing　in　our　lab.　©　2010　IEEE.