An　up-flow　anaerobic　sludge　blanket　(UASB)　reactor　was　operated　to　investigate　the　performance,　sludge　granulation,　and　microbial　community　dynamics　of　the　simultaneous　anammox　and　denitrification　(SAD)　process　at　different　C/N　ratios　by　inducing　a　shift　from　the　anammox　process.　The　nitrogen　removal　efficiency　(NRE)　increased　with　an　increase　in　the　C/N　ratio　at　lower　C/N　ratios　(≤　0.5).　The　NRE　and　nitrogen　removal　rate　(NRR)　were　90.81%　and　1.75　kgN　m−3d−1,　respectively,　at　a　C/N　ratio　of　0.5.　The　contribution　of　the　anammox　process　to　nitrogen　removal　(Eanammox)　decreased　sharply　to　51%　from　81%　as　a　result　of　the　decreased　activity　of　the　anaerobic　ammonia-oxidizing　bacteria　(AnAOB)　at　a　C/N　ratio　of　0.9.　The　EPS　production　and　the　ratio　of　the　protein　fraction　of　EPS　(EPSp)　to　the　carbohydrate　fraction　of　EPS　(EPSc)　increased　with　the　increase　in　the　C/N　ratio　(0.0–0.9),　which　induced　sludge　aggregation　and　resulted　in　larger　particle　size.　The　enrichment　of　denitrifying　bacteria　(DNB)　on　the　surface　of　the　AnAOB　due　to　EPS　bridging　further　enhanced　sludge　granulation.　The　Illumina　sequencing　results　showed　that　the　dominant　genus　of　AnAOB　(Candidatus　Jettenia)　initially　increased　with　an　increase　in　the　C/N　ratio　at　lower　levels　and　then　decreased　sharply　at　a　C/N　ratio　of　0.9.　The　relative　abundance　of　Candidatus　Jettenia　was　promoted　by　decreasing　the　C/N　ratio,　which　supported　the　recovery　of　the　AnAOB　activity.　Denitratisoma,　Bacillus,　Acinetobacter,　and　Pseudomonas　were　responsible　for　denitrification　throughout　the　entire　operational　period.　©　2020　The　Society　for　Biotechnology,　Japan