A new method for preparing high bending strength porous silicon nitride (Si3N4) ceramics with controlled porosity has been developed by using pressureless sintering techniques and phosphoric acid (H3PO4) as the pore-forming agent. The fabrication process is described in detail and the sintering mechanism of porous ceramics is analyzed by the X-ray diffraction method and thermal analysis. The microstructure and mechanical properties of the porous Si3N4 ceramics are investigated, as a function of the content of H3PO4. The resultant high porous Si3N4 ceramics sintered at 1000 degrees-1200 degrees C show a fine porous structure and a relative high bending strength. The porous structure is caused mainly by the volatilization of the H3PO4 and by the continous reaction of SiP2O7 binder, which could bond on to the Si3N4 grains. Porous Si3N4 ceramics with a porosity of 42%-63%, the bending strength of 50-120 MPa are obtained.