Abstract: Through varying the ultrasonic path length,the ultrasonic attenuation spectrum of the nanometer particle suspensions is measured.Based on the McClements and BLBL models,the Optimum Regulaxization Technique is employed to inverse the particle size distribution of nanometer particles.Experiments are carried out to measure the particle size distribution of aqueous nanometer titanium dioxide suspension with volume fraction of 1%.The ultrasonic measurement results show good consistence with those from TEM (Transmission Electron Microscope) and centrifugal sedimentation particle size analyzers,which indicate that measuring the particle size distribution of nanometer particles with the ultrasonic attenuation spectrum is feasible and reliable.