Pressure drop across filter materials is critical for the effective performance of various kinds of filtering system. To evaluate the feasibility of lowering the pressure drop of porous sintered reaction-bonded silicon nitrides, permeability tests were conducted for specimens with different porosity and pore structure. The results showed that pore former size is a minor factor, while permeability (k) had a strong dependency on the amount of pore former. Prolonged sintering time (2 h vs 10 h at 17001C) increased the permeability by increasing the effective pore diameter, which compensates for the observed decrease in total porosity.