The typical size distribution of emission particulates is bi-modal in shape with particles in the fine mode (<2.0 μm) and the coarse mode. An experimental study of pressure drop across industrial gas cleaning filters has been conducted using a particle mixture of fine alumina and coarse Arizona dusts with a rotating aerosol disperser to generate the bi-modal test aerosol. Pressure drop increased linearly with increasing mass loading. The pressure drop was found to be strongly dependent upon the mass ratio of fine to coarse particles. The measured specific resistances of HEPA filters at a given face velocity of 5 cm/s were 1.18×106, 5.89×105, 4.67×105, 2.65×105, and 1.18×105 s-1 for the mass ratio of fine to coarse particles of fine only, 50%:50%, 25%:75%, 10%:90%, and coarse particles only, respectively. The pressure drop across the loaded filter increased with increasing face velocity. The larger the mass ratio of fine to coarse particles and the higher the face velocity are, the faster pressure drop rises. The fine particles and the greater inertia of the particle moving fast would cause a denser cake formation on the filter surface, resulting in a greater specific resistance to the gas flow.