Ag-MgF_(2) composite films with different Ag fractions were prepared through a co-evaporation method. Microstructure analysis shows that the films are composed of amorphous MgF_(2) matrix and embedded fcc-Ag nanoparticles. The optical constants and their dispersion of the films, within the wavelength range of 250-650 nm, were measured by reflecting spectroscopic ellipsometry. The maximum of the imaginary part ε'' of the complex dielectric permittivity attributing to the surface plasmon resonance polarization of the Ag nanoparticles in an Ag-MgF_(2) film, and the tangent of the phase-shift angle δ resulting from the dielectric loss of the film, occur at λ=435 nm and λ=420 nm, respectively. Based on Maxwell-Garnett effective medium theory, the experimentally observed dispersion spectra were reasonably described.