Polymeric　carbon　nitride　often　displays　weak　photoluminescence　in　solid　state　due　to　the　aggregation-caused　quenching　effect.　Herein,　highly　fluorescent　carbon　nitride　oligomer　(CNO)　with　aggregation-induced　emission　(AIE)　characteristic　was　prepared　via　one-step　solid-phase　thermal　condensation　of　2,4-diamino-6-phenyl-1,3,5-triazine　(DPT)　at　350　°C.　CNO　is　mainly　composed　of　DPT　dimer　connected　by　rotatable　imine　groups,　and　exhibits　weak　fluorescence　in　the　dispersed　state　and　strong　blue-green　emission　in　the　aggregated　state　and　solid　state.　Density　functional　theory　calculations　indicate　that　the　restriction　of　phenyl　and　triazine　ring　twisting　motions　is　the　main　origin　of　the　AIE　phenomenon　of　CNO.　Finally,　CNO　was　preliminarily　applied　for　fluorescent　staining　of　plastic　pellets.　This　work　not　only　provides　a　solid-state　strategy　to　synthesize　fluorescent　material　with　AIE　characteristic　but　also　extends　the　application　of　polymeric　carbon　nitride.　©　2022　Elsevier　B.V.