Even　though　the　membrane　biofilm　reactor　(MBfR)　performing　the　anammox　and　nitrate/nitrite-dependent　anaerobic　methane　oxidation　(n-DAMO)　integrated　processes　has　been　known　to　enable　complete　nitrogen　removal,　the　effects　of　biofilm　properties　on　such　an　MBfR　are　yet　to　be　disclosed.　In　this　work,　a　biofilm　model　was　constructed　to　investigate　the　effects　of　the　initial　microbial　composition　of　the　biofilm,　the　initial　biofilm　thickness,　the　boundary　layer　thickness　of　the　biofilm,　and　the　diffusivity　of　solutes　in　the　biofilm　structure　on　the　start-up　process　and　steady-state　performance　of　the　MBfR　performing　anammox/n-DAMO.　The　results　showed　that　the　four　biofilm　properties　would　not　affect　the　steady-state　performance　but　would　significantly　regulate　the　start-up　time　of　the　MBfR.　Unless　the　MBfR　was　operated　under　undesired　operational　conditions,　inoculation　of　sludge　comprised　mainly　of　anammox　bacteria　or/and　n-DAMO　archaea　to　form　a　thin　initial　biofilm　would　accelerate　the　start-up　process　of　the　MBfR.　Moreover,　measures　could　be　taken　to　reduce　the　boundary　layer　thickness　and　the　diffusivity　of　solutes　in　the　biofilm　structure,　the　latter　of　which　would　also　enhance　methane　utilization.　This　work　would　provide　valuable　practical　guidance　for　the　rapid　establishment　of　the　MBfR　with　a　high-level　treatment　capacity　based　on　anammox/n-DAMO.　©　2023　American　Chemical　Society.