Wepresent a novel method for the preparation of ultrasmall Au/CdSe core/shell particles. Au-Cd bialloy particles of 4.7nm diameter were prepared as the precursor. The Cd component in the precursor reacted with the Se source at a temperature of 205℃ and was heated to 250℃, leading to formation of a Au/CdSe core/shell structure. The sizes of Au/CdSe nanoparticles have a narrow distribution with an average size of 6.0nm and Au core of 2.2nm diameter. The X-ray diffraction pattern and the images of the highresolution electron transmission microscopy show that the Au cores and the CdSe shells of Au/CdSe core/shell nanoparticles are both well crystallized, and the CdSe shells are in a cubic phase. The absorption spectrum of the Au/CdSe nanoparticles combines the absorption behaviors of the Au cores and the CdSe shells.