Uniform CdS colloidal spheres have been successfully synthesized via a simple hydrothermal method. X-ray diffraction (XRD) analyses indicate that the products exhibit a hexagonal structure. Scanning electron (SEM) and transmission electron microscopy (TEM) are used to characterize CdS colloidal spheres. The final size of the spheres may be selected from a range of 71 ± 2 nm to approximately 181 ± 5 nm by changing the amount of polyvinylpyrrolidone (PVP) and hexamethylenetetramine (HMT). The CdS colloidal spheres are not obtained in the absence of either of the capping agents. A synergistic effect between HMT and PVP is proposed to be crucial for the formation of colloidal spheres.