Porous　silicon　nitride　ceramics　with　high　porosity　and　uniform　pore　structure　were　prepared　by　carbothermal　reduction　reaction　between　low-cost　silicon　dioxide　and　activated　carbon.　The　influence　Of　SiO2　particle　sizes　on　their　microstructures　was　investigated.　Microstructures　and　mechanical　properties　of　porous　silicon　nitride　ceramics　were　studied　by　XRD,　SEM　and　three-point　bending　measurement.　XRD　analysis　showed　that　only　beta-Si3N4　phase,　minor　of　alpha-Si3N4　phase　and　glass　phase　Y8Si4N4O14　were　detected.　SEM　analysis　showed　that　porous　silicon　nitride　ceramics　obtained　were　composed　of　rod-like　beta-Si3N4　grains　and　uniform　pores.　Porous　silicon　nitride　ceramics　with　different　porosities　and　good　mechanical　properties　were　fabricated　by　changing　SiO2　particle　sizes.