The effect of pore former content on both porosity and pore structure was investigated for porous sintered reaction-bonded silicon nitrides (SRBSNs). A spherical PMMA with $d_{50}=8{mu}m$ was employed as a pore-former. Its amount ranged from 0 to 30 part. Porous SRBSNs were fabricated by post-sintering at various temperatures where the porosity was controlled at $12{sim}52%$. The strong tendency of increasing porosity with PMMA content and decreasing porosity with sintering temperature was observed. Measured pore-channel diameter increased $(0.3{ ightarrow}1.1{mu}m)$ with both PMMA content and sintering temperature.