Dissolved　organic　nitrogen　(DON)　is　a　component　of　wastewater　with　a　negative　influence　on　the　environment.　The　removal　of　DON　is　conducted　through　the　anoxic/oxic　(A/O)　and　anammox　processes.　However,　the　mechanisms　and　chemical　preferences　in　the　removal　of　DON　compounds　have　not　been　understood　and　compared　so　far.　This　study,　for　the　first　time,　comparatively　investigated　the　molecular-level　characteristics　of　DON　during　both　processes　by　using　FT-ICR　MS　(Fourier　transform　ion　cyclotron　resonance　mass　spectrometry).　The　results　indicated　that　the　number　of　DON　formulas　increased　from　1844　to　1935　during　A/O　process,　and　from　2784　to　3242　during　anammox　process,　highlighting　the　increase　in　complexity　of　DON　after　undergoing　both　processes.　DON　with　high　saturation　and　aliphatic　structures　were　removed　by　A/O　process,　whereas　highly　unsaturated　and　aromatic　structures　were　removed　by　anammox　process.　For　DON　without　S　atom,　Lignin-like　and　tannin-like　ones　were　resistant　to　both　processes　and　protein-like　and　condensed　aromatic　structures　were　resistant　to　anammox　process.　The　complementarity　of　these　two　processes　provided　a　sequential　combination　with　sufficient　theoretical　support　to　improve　DON　removal　efficiency.