Si3N4　ceramic　with　ultrafine　fibrous　grains　are　expected　to　exhibit　remarkable　mechanical　properties.　In　this　work,　highly　porous　Si3N4　ceramic　monoliths　composed　of　ultrafine　fibrous　grains　were　developed　via　a　novel　vapor-solid　carbothermal　reduction　nitridation　(V-S　CRN)　reaction　between　SiO　vapor　and　green　bodies　comprised　of　carbon　nanotubes　(CNTs),　α-Si3N4　diluents　and　Y2O3　in　a　N2　atmosphere.　The　unique　fibrous　grains-interconnected　structure　was　developed　through　in-situ　formation　of　Si3N4　and　following　liquid　phase　sintering.　The　porous　Si3N4　monoliths　with　porosity　of　61–78%　was　developed　by　controlling　the　contents　of　α-Si3N4　diluents　and　densities　of　the　CNT　green　bodies.　With　increasing　of　the　α-Si3N4　contents,　Si3N4　fibrous　grains　with　an　aspect　ratio　of　approximate　or　higher　than　20　could　be　achieved,　and　the　grains　were　gradually　refined.　For　the　samples　with　40　wt%　α-Si3N4,　the　minimum　mean　grain　diameter　and　pore　size　of　164　nm　and　0.79　μm　were　achieved,　respectively,　and　the　resultant　porous　Si3N4　monolith　exhibited　a　flexural　strength　of　as　high　as　73–102　MPa　with　the　porosity　of　61–73%,　which　is　much　higher　than　that　of　the　reported　in　literature.　The　improvement　of　mechanical　strength　could　be　attributed　to　the　densely　interconnected　bird＇s　nests　structure　formed　by　the　ultrafine　fibrous　grains.　The　effects　of　the　α-Si3N4　diluents　on　the　resulting　porous　Si3N4　monolith　via　this　method　were　analyzed.　©　2020