The particle size distribution of silica powders in diluted dispersed suspension was evaluated by ultrasonic attenuation spectroscopy and compared to that analyzed by X-ray sedimentation. With increasing diameter of the silica particle, the ultrasonic attenuation was dominantly influenced by viscous losses as well as scattering losses. It was shown that the calculated particle distribution based on measurement of these two losses was in good agreement with that obtained by X-ray sedimentation.