Porous　Si3N4/25%BN(in　volume,　h-BN)　composite　ceramics　were　fabricated　by　pressureless　sintering　using　mi-cron-sized　α-Si3N4　and　h-BN　powders　as　starting　materials　and　Yb2O3,　Al2O3　and　Y2O3　as　sintering　additives.　The　effects　of　the　Yb2O3　content　on　the　mechanical　properties　of　the　Si3N4/25%BN　composites　were　investigated,　and　the　phase　composition　and　microstructure　of　the　Si3N4/25%BN　multiphase　composite　ceramics　were　analyzed　by　X-ray　diffraction　and　scanning　electron　microscopy.　The　experimental　results　show　that　as　Yb2O3　content　increases,　the　porosity　of　Si3N4/BN　composite　ceramics　increases,　and　its　shrinkage　and　relative　density　decrease.　The　apparent　porosity　of　composites　increases　from　15.1%　to　32.1%,　and　its　relative　density　decreases　from　72.8%　to　60.3%　with　the　increase　of　Yb2O3　content　in　mass　from　2%　to　15%.　The　flexural　strength　of　the　samples　fistly　increases　and　then　decreases　as　the　amount　of　Yb2O3　increases.　When　the　amount　of　the　Yb2O3　is　4%　in　mass,　the　Si3N4/25%BN　composite　ceramic　has　the　highest　flexural　strength　reaching　to　264.3　MPa　under　room-temperature.