In the present paper, CdS nanoparticles of different size surface-capped with thiourea [expressed as CdS/SC(NH2)2] were prepared by sol-gel method, and characterized by XRD, TEM, IR and PL. The results show that the molar ratio of thiourea to cadmium ion has influence on the size and luminescence properties of the nanoparticles. With increasing the mdar ratio of thiourea to cadmium ion in the reactant, the size of the nanoparticles decreases, and the maximum emission wavelength of the nanoparticles shows an obvious blue shift due to quantum confinement effect. The effect of calf thymus deoxyribonucleic acid (ct-DNA) on the fluorescence spectrum of the CdS/SC(NH2)2 nanoparticles was also studied. The result shows that the intensity of fluorescence spectrum of the nanoparticles is quenched by ct-DNA, which is probably caused by the electrostatic interaction between CdS/SC(NH2)2 nanoparticles and ct-DNA. The CdS/SC(NH2)2 nanoparticles are hopeful of being used as fluorescence probes in DNA determination.