Process fluids may be degraded by large concentrations of suspended particles, formations of gel slugs, the infiltration of air bubbles, and so on. The scattering of ultrasound is dependent on the relation of the wavelength and the scatterers' dimensions. Relatively large objects are easily detected because of their strong scattering in the short wavelength limit (SWL), but small ones may be missed because of their weak scattering in the long wavelength limit (LWL). When the object dimension is comparable with the wavelength of ultrasound (intermediate region), the approximations of both the SWL and the LWL do not apply. We experimentally investigated the scattering characteristics of particles in the intermediate region to estimate the size distribution of particles in some industrial fluids. Observed particle size distributions are studied and an exponential regression approach is proposed. The results demonstrate that moving scatterers detected by ultrasound are able to be recognized in real time by the system, and their sizes can be individually determined as well as their statistical distribution in each trial period. Finally, rules have been found to estimate the original distribution of particles from a derived system parameter and observed data.