Cu-MgF2 composite nanoparticle films with the copper volume fraction of 15% and 30% were prepared by radio-frequency magnetron co-sputtering technique. The transmission electron microscopy (TEM) images of Cu-MgF2 composite films show that the films are mainly made of amorphous ceramic MgF2 matrix with embedded Cu nanoparticles with various grain shapes. The optical constants spectra of the composite films are determined as a function of wavelength in the range of 270～830 nm by spectroscopic ellipsometry. The theoretical optical constants spectra of the composite films are calculated in the same wavelength range by effective medium approximation in which grain shape effect is incorporated. The optical properties of the two composite films are analyzed and discussed by combining TEM images with experimental and theoretical optical constants spectra. The conclusion is drawn that Maxwell-Garnett model in which the depolarization factor of Cu grains is 0.33 is suitable to explain the optical properties of 15% Cu-MgF2 film, and the optical properties of 30% Cu-MgF2 film may be better interpreted with the depolarization factor of Cu grains 0.6.