This thesis presents a theoretical study on the influence of size and environment on the optical response of bimetallic spherical nanoparticles for core/shell structure of Au/Ag system in a large spectral range (λ=300-900 nm). The optical spectra of noble metallic nanoparticles exhibit an intense resonance band in the near UV – visible spectral range, the well-known surface plasmon resonance (SPR), whose characteristics (spectral position, magnitude and width) are strongly sensitive to intrinsic properties (size, shape, structure and local environment). Numerical calculations based on dipolar approximation (DA) for nanoparticles of sizes (D=10-40 nm) were developed in order to describe the extinction (absorption and scattering) spectra of bimetallic nanoparticles structure. The obtained results show that the shell composition characters play an important role on the optical properties of bimetallic nanoparticles. For gold spherical nanoparticles the wavelength position corresponding to maximum extinction shows a no shifting by increasing the size of the nanoparticle. When Au nanoparticles was coated by an Ag shell, a red shift was observed by increasing the Au core radius. However with increasing the thickness of silver, the SPR spectrum shifts to shorter wavelengths. This blue shift is accompanied by an increase in peak intensity. Moreover, red shift in SPR band has appeared with increasing the dielectric constant of the surrounding medium of the bimetallic structure.