The combining of metal and semiconductor nanostructures as core/shell have attracted great interest due to their complementary optical properties; the localized Plasmon in the core and the exciton in semiconductor. The first, results in the enhancement of local fields and the second, are the nonradiative damping due to electron transfer from semiconductor to metal. Composite nanostructures of metal-core/semiconductor-shell configuration were prepared using organometalic method. UV-Vis spectra for the colloids show two distinct bands, one for the gold (Au) metal core surface plasmon at lower energy and the second for the excitons of semiconductor Cadmium Sulfide (CdS) shell at higher energy. Second derivative methods were used to determine the precise absorption peaks in UV-Vis spectra. The shell size was estimated using two methods, effective mass approximation (EMA), and polynomial fitting function (PFF), both give shell thickness dimension in the range of few nanometers. These dimensions were confirmed by imaging the nanostructure using high resolution transmission electron microscope (HRTEM).