A simple sonochemical route for the surface coating of titanium dioxide on cadmium sulfide nanocrystal was reported.After 2 h ultrasonic irradiation treatment,the mixture of CdS nanocrystals and tetrabutyl titanate in an aqueous medium yielded CdS/TiO_2 nanocrystals composites with core/shell structure.The thickness of TiO_2 layer with smooth interface could be easily controlled via changing the concentration of the precursors and the time of irradiation.The core/shell nanocrysrals were characterized by X-ray diffraction,transmission electron microscope and UV-vis spectrometry techniques.The prepared semiconductor composites with particular band structure present appealing properties especially in photochemical activity.