There have been great interests in the optical, electrical, and chemical properties of quantized nanoparticles, such as semiconductor quantum dots (QDs). 1 Semiconductor QD structures particularly have attracted attentions because of their strong three dimensional quantum confinement effects. Researches have been held to develop better methods to fabricate such semiconductor QDs, showing great progress during the past decade. One of the most early developed fabrication methods is to embed II-VI semiconductor compound QDs into glass matrices. 2,3 However, recently, many research groups have established chemical fabrication methods of colloidal QDs dispersed in solution, which are comparatively more stable than those in glasses. 4-6 In particular, a radiolysis method utilizing a gamma ray source at room temperature has been actively and successfully conducted, which allows us to monitor the size of the sample during each fabrication process. 7-9 These colloidal QDs are produced by chemical reactions in equilibrium states, where the solution matrices act as a stabilizer, offering fast reaction time between the atoms as well as a slow growth rate during the fabrication procedure, helping to improve stability. In general, capping ligand is usually added to avoid aggre-gation of the nanoparticles and increase the solubility. Along with semiconductor QDs, solid metal nanoparticles also display distinctive and potentially productive properties. Characteristics of surface enhanced Raman scattering and surface second harmonic generation due to local field enhancement associated with plasmon resonance are well known examples of such the specialties. 10-12 Metal nano-particles also have a large and fast third order optical nonlinearity opening possibilities for optical switching devices. 12 Fabrication methods of metal nano-structured particles, like in the case of semiconductor nanoparticles, have been continuously improved to reduce inhomogeneous broadening, optimizing their optical properties. If the metal nano-structure is incorporated into the QD system, coupling between the plasmon resonance effect and the quantum size effect of the semiconductor QD may develop new aspects of nano-composite material systems and also widen applications for noble nano-devices. 13-17 In this article, the fabrication and optical characteristics of CdS/ Ag semiconductor-metal composite QD structures are presented. The CdS QDs were made of cadmium sulfate and 2-mercaptoethanol by gamma ray irradiation in aqueous solution and silver was partially covered around the fabricated CdS QDs. The measured absorption spectra showed exciton peaks due to the quantum confinement effect as well as the surface plasmon resonance effect. 18,19 A red-shift of the CdS exciton absorption peak was observed, strongly indicating an enhancement of the local field in the semiconductor core. High Resolution Transmission Electron Microscopy (HRTEM) was also carried out to verify formation of CdS QDs and CdS/Ag composite QDs.