Porous Si3N4 -Si2N2O bodies fabricated by multi-pass extrusion process were investigated depending on the feldspar addition content (4-8 wt% Si) in the raw silicon powder. The diameter of the continuous pores was about 250 mu m. The polycrystalline Si2N2O fibers observed in the continuous pores as well as in the matrix regions of the nitrided bodies can increase the filtration efficiency. In the 4 wt% feldspar addition, the diameter of the Si2N2O fibers in the continuous pores of the nitrided bodies was about 90-150 nm. A few number of rope typed Si2N2O fibers (similar to 4 mu m) was found in the case of 8 wt% feldspar addition. However, in the 8 wt% feldspar addition, the matrix showed highly porous structure composed of large number of the Si2N2O fibers (similar to 60 nm). The relative densities of the Si3N4-Si2N2O bodies with 4 wt% and 8 wt% feldspar additions were about 65% and 61%, respectively.