The　nitrite　accumulation　in　the　denitrification　process　is　investigated　with　sequencing　batch　reactor　(SBR)　treating　pre-treated　landfill　leachate　in　anoxic/anacrobic　up-flow　anaerobic　sludge　bed　(UASB).　Nitrite　accumulates　obviously　at　different　initial　nitrate　concentrations　(64.9,54.8,49.3　and　29.5　mg.L-1)　and　low　temperatures,　and　the　two　break　points　on　the　oxidation-reduction　potential　(ORP)　profile　indicate　the　completion　of　nitrate　and　nitrite　reduction.　Usually,　the　nitrate　reduction　rate　is　used　as　the　sole　parameter　to　characterize　the　denitrification　rate,　and　nitrite　is　not　even　measured.　For　accuracy,　the　total　oxidized　nitrogen　(nitrate　+　nitrite)　is　used　as　a　measure,　though　details　characterizing　the　process　may　be　overlooked.　Additionally,　batch　tests　are　conducted　to　investigate　the　effects　of　C/N　ratios　and　types　of　carbon　sources　on　the　nitrite　accumulation　during　the　denitrification.　It　is　observed　that　carbon　source　is　sufficient　for　the　reduction　of　nitrate　to　nitrite,　but　for　further　reduction　of　nitrite　to　nitrogen　gas,　is　deficient　when　C/N　is　below　the　theoretical　critical　level　of　3.75　based　on　the　stoichiometry　of　denitrification.　Five　carbon　sources　used　in　this　work,　except　for　glucose,　may　cause　the　nitrite　accumulation.　From　experimental　results　and　cited　literature,　it　is　concluded　that　Alcaligene　species　may　be　contained　in　the　SBR　activated-sludge　system.