Cadmium sulfide (CdS) nanoparticles of about 3 nm in diameter have been prepared by the sonochemical reduction of a mixed solution of cadmium chloride (CdCl 2 ), sodium thiosulfate (Na 2 S 2 O 3 ) and isopropyl alcohol ((CH 3 ) 2 CHOH) in Ar atmosphere at room temperature. The CdS nanoparticles were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The formation mechanism of CdS nanoparticles is also discussed together with the optical absorption spectroscopy analysis. The method presented here can be used for the preparation of other sulfide nanoparticles.