Abstract: CdS nanocrystals synthesized in aqueous solution were used as a kind of fluorescence probe to label Bovine Serum Albumin （BSA）. After being purified by a SephadexG-25 column, the luminescence of the CdS-BSA composite can be visualized under an ultraviolet lamp. Fluorescent spectral studies show that the fluorescence intensity of the CdS nanocrystals experiences little changes after being coupled with the BSA.