The present paper describes the feasibility of controlling pore sizes of porous-fired Si3N4 compacts prepared using the pyrolysis of polysilazane, for developing a Si3N4 membrane manufacturing technique. Attention has been paid to the microstructure of these compacts in order to identify the main parameter governing the evolution of polysilazane-to-ceramic direct conversion. A volumetric shrinkage, stemming from the pyrolysis of polysilazane and the following crystallization of pyrolyzed polysilazane, have been shown to be of prime interest in the control of pore-size determination. A porosity of similar to 35% was achieved in the fabrication of compacts, the pore-size distribution of which was controlled in the range 1-1000 nm, with polysilazane additions of 80-200 vol%. Also demonstrated was the effect of firing temperature on pore-size distribution. The bending strength of the processed specimens was equivalent to that of Vycor glass, indicating the feasibility of an application in the preparation of microstructured membrane system.