One-dimensional　Si3N4　nanostructures　are　desirable　for　constructing　nanoscale　electric　and　optoelectronic　devices　due　to　their　peculiar　morphologies.　Herein,　a　facile　and　environmentally　friendly　catalyst-free　method　is　proposed　to　synthesize　ultra-long　single　crystal　α-Si3N4　nanobelts　via　carbothermal　nitridation　of　carbon　nanotubes　at　1750　C.　The　obtained　α-Si3N4　nanobelts　with　a　flat　surface　(thickness　of　∼150　nm,　length　of　several　millimeters)　exhibited　an　extremely　high　aspect　ratio,　perfect　crystal　structure,　and　high　specific　surface　area　of　7.34-10.09　m2　g-1.　In　addition,　the　width　was　increased　from　approximately　80　nm　to　8　μm　by　increasing　the　holding　time　from　1　to　3　h.　The　nanobelt　formation　was　governed　by　the　vapor-solid　(VS)　reaction　between　SiO　vapor,　N2　and　carbon　nanotubes,　and　the　vapor-vapor　reaction　between　SiO,　CO　and　N2.　The　former　was　responsible　for　the　initial　nucleation　and　successive　base-growth　of　α-Si3N4　nanotubes.　The　latter　additionally　contributed　to　the　nanorod　and　subsequent　proto-nanobelt　formation　and　to　the　growth　of　the　nanobelts.　During　high-temperature　annealing　at　1750　C,　the　original　Si3N4　nanotubes　gradually　transformed　into　nanorods,　and,　finally,　nanobelts　with　stable　shapes　as　a　result　of　surface　energy　minimization.　©　2020　IOP　Publishing　Ltd.