Porous　silicon　nitride　ceramics　with　various　amounts　(25,　35,　and　45　vol　%)　of　hexagonal　boron　nitride　(h-BN)　were　fabricated　at　1800　degrees　C　for　2h　by　the　pressureless　sintering　process.　With　FESEM　and　TEM,　the　effects　of　h-BN　on　the　microstructure　and　mechanical　properties　of　Si3N4　ceramics　were　investigated.　Results　of　the　microstructure　and　mechanical　properties　of　Si3N4/BN　composites　showed　that　the　growth　of　the　elongated　beta-Si3N4　were　hindered　by　h-BN　additive,　which　resulted　in　the　decrease　of　fracture　toughness　of　Si3N4/BN　ceramics　with　increasing　h-BN　content.　The　morphologies　of　the　fracture　surfaces　by　FESEM　revealed　the　fracture　mode　for　Si3N4/BN　composites　to　be　intergranular.　However,　phase　analysis　by　XRD　indicated　that　the　effect　of　h-BN　on　the　alpha-　to　beta-　Si3N4　phase　transformation　of　Si3N4/BN　composites　was　negligible.