Carbon　nanotubes　(CNTs)　as　well-known　nanomaterials　are　extensively　studied　and　widely　applied　in　various　fields.　Nitric　acid　(HNO3)　is　often　used　to　treat　CNTs　for　purification　purposes　and　preparing　oxidized　CNTs　for　various　applications.　However,　too　little　attention　is　paid　to　investigating　the　effect　of　HNO3　treatment　on　the　optical　properties　of　CNTs.　In　this　work,　it　is　observed　for　the　first　time　that　HNO3-oxidized　multiwalled　carbon　nanotubes　(ox-MWCNTs)　have　strong　electrochemiluminescence　(ECL)　activity,　which　enables　ox-MWCNTs　to　become　new　and　good　ECL　carbon　nanomaterials　after　carbon　quantum　dots　(CQDs)　and　graphene　quantum　dots　(GQDs).　Various　characterization　technologies,　such　as　transmission　electron　microscope　(TEM),　X-ray　photoelectron　spectroscopy　(XPS),　and　Raman　spectroscopy,　are　used　to　reveal　the　relationship　between　ECL　activity　and　surface　states.　The　ECL　behaviors　of　ox-MWCNTs　are　investigated　in　detail　and　a　possible　ECL　mechanism　is　proposed.　Finally,　the　new　ECL　nanomaterials　of　ox-MWCNTs　are　envisioned　to　have　promising　applications　in　sensitive　ECL　sensing　and　in　the　study　of　CNT-based　catalysts.　©　2019　WILEY-VCH　Verlag　GmbH　&　Co.　KGaA,　Weinheim