Nitrite　accumulation　during　the　denitration　process　with　SBR　treating　pre-treated　by　anoxic/anaerobic　up-flow　anaerobic　sludge　bed　(UASB)　was　observed　at　low　temperatures.　The　effects　of　types　of　carbon　sources　on　nitrite　accumulation　were　investigated　for　comprehensive　understanding　the　mechanism　of　nitrite　accumulation.　Experimental　results　clearly　showed　that　nitrite　accumulated　obviously　when　five　carbon　sources　(methanol,　ethanol,　sodium　acetate,　sodium　propinoate　and　glucose),　except　for　glucose,　were　used　as　electron　donor.　Additionally,　nitrite　accumulations　were　observed　at　four　different　initial　nitrate　concentrations　and　lower　temperatures.　The　maximum　concentrations　of　nitrite　accumulation　were　37.8,　21.5,　25.2　and　18.8　mg/L,　respectively,　and　the　corresponding　nitrite　accumulation　rates　(N/VSS　x　t)　were　0.117,　0.136,　0.235　and　0.068　g/(g　x　d)　during　the　nitrite　accumulation　period.　Two　break　points　of　＂nitrate　knee＂　and　＂nitrite　knee＂　on　oxidation　reduction　potential　(ORP)　profile　indicated　that　the　reduction　reactions　of　nitrate　and　nitrite　were　completed,　respectively.