The ellipsometric data in near and mid infrared waveband are fitted by double Lorentz and effective medium approximation (EMA) dispersion models for annealed Mn1.95Co0.77Ni0.28O4thin films. By comparing the tested conductivity under direct current (DC) voltage with the calculated conductivity at low frequency (ω→0), it is found that A oscillator mainly generates conduction current and B oscillator mainly generates displacement current. For Mn1.95Co0.77Ni0.28O4thin films, the lattice resonance absorption will gradually be replaced by the electron resonance absorption with increasing annealing temperature. Both the refractive index n and extinction coefficient k of the annealed samples are determined by the annealing process.