In　the　present　study,　the　heterotrophic　nitrification-aerobic　denitrification　strain,　Acinetobacter　sp.　JQ1004,　was　treated　with　three　typical　N-acyl-homoserine　lactone　(AHL)　molecules　(C6-HSL,　C8-HSL,　and　3-oxo-C10-HSL)　during　the　nitrogen　removal　process.　The　effects　of　AHLs　on　biofilm　formation,　flocculation,　extracellular　polymeric　substance　characteristics,　and　nitrogen　removal　were　investigated.　Findings　revealed　that　low　concentrations　of　these　three　AHLs　could　promote　ammonia　removal　and　cell　growth　as　follows:　C8-HSL　＞　C6-HSL　＞　3-oxo-C10-HSL,　whereas　high　concentrations　suppressed　nitrogen　removal.　Transcript　levels　of　the　amoA　gene　in　the　heterotrophic　nitrification　process　were　detected　by　real-time　PCR,　indicating　that　the　addition　of　each　AHL　with　10　nmol　L-1　could　stimulate　expression　of　amoA.　Notably,　the　addition　of　C6-HSL　at　relative　lowly　concentrations　significantly　accelerated　biofilm　formation　and　self-aggregation　of　strain　JQ1004.　Many　microbial-flocs　were　observed　between　cells　using　scanning　electron　microscopy　when　strains　were　dosed　with　10　nmol　L-1　C6-HSL.　Excitation　emission　matrix　spectra　revealed　that　the　addition　of　C6-HSL　and　C8-HSL　at　10　nmol　L-1　did　not　change　the　components　and　structures　of　the　extracellular　polymeric　substance　(EPS),　but　the　fluorescence　intensity　of　various　components　increased　substantially.　However,　the　addition　of　3-oxo-C10-HSL　reduced　the　fluorescence　strength　of　EPS,　which　had　no　remarkable　effect　on　biofilm　formation,　self-aggregation,　and　nitrogen　removal　of　the　strain.