Magnetic liquids are stable colloidal suspensions of nano-sized magnetic particles in a carrier liquid medium. In the present paper the determination of the particle size distribution function using ultrasonic spectroscopy is described. The ultrasonic spectra of water-based magnetic fluid measured in the 3.5-50 MHz frequency range are analyzed using formulas for the velocity and absorption of sound in dispersion media obtained by Vinogradov. The results of the ultrasonic studies are compared with the particles size distribution function evaluated from the processing of the magnetic susceptibility data.