Well-defined CdS nanorings with flower-like morphology were synthesized by a hydrothermal method using poly(vinyl-pyrrolidone) as capping agent. The phase composition, morphology, structure, and optical properties of CdS nanorings were characterized by X-ray diffraction (XRD), scanning electron microscope, transmission electron microscopy, and UV-vis absorption spectroscopic techniques. The XRD pattern of the sample can be indexed to the cubic zinc blende phase CdS. According to the quantitative analysis of energy-dispersive spectrum, the Cd:S molar ratio of the sample is about 1:0.96. The possible formation mechanism of the CdS nanorings is proposed which is based on time-resolved experiments. Furthermore, the absorption peak of CdS nanorings is red-shifted to 523 nm in the UV-vis absorption spectrum.