Achieving　the　effective　retention　and　rapid　enrichment　of　anammox　bacteria　(AnAOB)　remains　a　challenge　in　landfill　leachate　treatment　systems.　The　application　of　the　anammox　process　is　also　restricted　by　an　inadequate　supply　of　nitrite　and　poor　resistance　to　adverse　environments.　To　overcome　these　obstacles,　an　up-flow　dual-loop　anammox　(UDLA)　system　treating　mature　landfill　leachate　was　established,　based　on　partial　nitrification,　anammox,　and　partial　denitrification　(PNAPD)　technology.　The　nitrogen　removal　efficiency　and　nitrogen　loading　rate　reached　92.9　±　1.5%　and　0.4　kg　N/m3/d,　and　isotope　tracing　tests　demonstrated　that　88.2　±　6.5%-93.9　±　3.9%　of　the　nitrogen　loss　contributed　by　anammox　pathway.　Under　the　up-flow　dual-loop　strategy,　anammox　granules　were　successfully　cultivated　when　flux　ratios　of　effluent/influent　and　produced　gas/influent　were　maintained　at　1640.0%,　and　250.0%,　respectively.　The　proportion　of　medium　granules　(500–1000　μm)　with　the　highest　PNAPD　activity　increased　from　0　to　24.9%,　whereas　the　percentage　of　floc　(0–200　μm)　reduced　from　98.4%　to　23.3%.　The　mechanism　of　anammox　granules　formation　in　the　UDLA　system　was　revealed　by　multidimensional　analysis.　Candidatus_Brocadia_sapporoensis　(13.3%-13.5%),　Candidatus_Kuenenia_stuttgartiensis　(2.0%-4.8%),　and　Candidatus_Brocadia_sp.　(0.9%-1.1%)　were　the　predominant　anammox　species,　enriched　in　medium　and　macro　granules.　Metagenomic　sequencing　confirmed　the　functional　divergence　of　nitrogen　and　carbon　metabolism　among　the　size-fractioned　granules,　possible　functions　of　specific　species,　and　gene　distribution　in　the　nitrogen　metabolism　pathway.　The　abundant　GlnA,　GltB(D),　GdhA,　GudB　(GDH2),　and　GLUD1_2　genes　might　promote　the　synthesis　of　adhesive　proteins　and　further　facilitate　microbial　aggregation.　This　study　enhanced　the　comprehension　of　anammox　granules　formation　and　showed　important　implications　for　leachate　treatment　in　engineering.　©　2023　Elsevier　B.V.