Silicon nitride (Si₃N₄) whiskers were coated on a porous Si₃N₄ substrate, which was coated with SiO₂-Al₂O₃-Y₂O₃ oxides, by gas phase processing using amorphous Si₃N₄ and TiO₂ powders as source materials. Coating was carried out in the interval 1350-1550°C in N₂. At temperatures below the melting point of the oxides, α-Si₃N₄ whiskers of high aspect ratio were coated with weak adhesion to the substrate. At temperatures above the melting point, β-Si₃N₄ whiskers with diameters of 0.02 to 0.07μm (low aspect ratio) were coated with strong adhesion to the substrate. When the Si₃N₄ substrates coated with the β-Si₃N₄ whiskers were used as membrane filters, it could separate particles as fine as 0.05μm in diameter.