The　start-up　performance　of　the　simultaneous　anammox　and　denitrification　(SAD)　process　was　studied　in　an　upflow　anaerobic　sludge　blanket　(UASB)　reactor　with　an　effective　volume　of　10　L.　The　activity　of　heterotrophic　denitrifying　bacteria　(DNB)　increased　when　the　C/N　ratio　was　0.25　over　53　d.　Ammonia　removal　efficiency　(ARE),　chemical　oxygen　demand　removal　efficiency　(CRE),　nitrogen　removal　efficiency　(NRE),　and　nitrogen　removal　rate　(NRR)　reached　93.27%　+/-　0.20%,　82.25%　+/-　0.04%,　82.39%　+/-　0.57%　and　0.90　+/-　0.02　kg/(m3　center　dot　d),　respectively.　A　kinetic　model　of　the　biological　reaction　system　of　the　SAD　process　start-up　was　developed　based　on　the　experimental　data　and　a　modified　mathematical　model　(ASM1).　The　effectiveness　of　the　model　was　verified　based　on　the　experimental　data,　and　the　nitrogen　removal　performance　and　changes　in　functional　bacteria　of　the　coupled　system　under　different　C/N　ratios　were　investigated.　The　simulation　revealed　that　the　concentration　and　activity　of　anaerobic　ammonia-oxidizing　bacteria　(AnAOB)　gradually　decreased,　and　DNB　gradually　increased,　as　the　C/N　ratio　increased　from　0.3　to　1.0.　The　optimal　synergy　of　AnAOB　and　DNB　and　a　stable　NRE　of　85.36%　+/-　0.48%　were　achieved　when　the　C/N　ratio　was　0.6.　Under　this　condition,　the　contributions　of　the　anammox　and　denitrification　processes　to　nitrogen　removal　(Eanammox　and　Edenitrification)　were　88.33%　+/-　0.23%　and　11.67%　+/-　0.23%,　respectively.