The ellipsometric data in near and mid infrared waveband are fitted by double Lorentz and effective medium approximation (EMA) dispersion models for annealed Mn_1.95Co_0.77Ni_0.28O₄ thin 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 Mn_1.95Co_0.77Ni_0.28O₄ thin 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.