CdS nanoparticles with good crystallinity were prepared by hydrothermal method in microemulsion composed of polyoxyethylene laurylether/water/cyclohexane/butanol.The structure and the size of the CdS nanoparticles were analyzed by TEM and XRD.The UV-Vis optical absorption of the samples was also investigated.The results show that hydrothermal treatment is an effective method to prepare CdS nanoparticles of hexagonal structure at lower temperature.The particles were in dimensional uniformity.The diameter of the CdS nanoparticles decreased with the increase of the molar ratio of water to surfactant.The minimum diameter of the CdS nanoparticles prepared in this work was about 10 nm.Obvious blue shift appeared in the UV-Vis absorption spectra.