A new foaming route, which was imitated from the foaming of aerated foods, was used by the protein foaming method in preparing highly porous silicon nitride (Si 3 N 4 ) foams. Effects of air fraction on the structure and property of the products were investigated. It was indicated that the porosity increased with the air fraction exponentially, whereas the air fraction had little effect on the average pore sizes of Si 3 N 4 foams. Thermal conductivity of the product measured using the laser flash method decreased from 4.07 to 2.29 W/(mÁK) when the porosity increased from 80.00% to 87.61%.