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Optical properties and electronic structures of VO2 films obtained by magnetron sputtering method on quartz and sapphire substrates were analyzed by transmission spectrum and spectroscopic ellipsometry (SE). The metal-to-insulator transition (MIT) characteristics of the two thin films were compared and discussed correlating with their structural details. By fitting spectroscopic ellipsometric data, the dielectric functions, electronic transitions, and optical bandgap of VO2 films can be tuned by substrate effects. On quartz substrate, the VO2 film exhibits a lower transition temperature and broader thermal hysteresis (ΔH). It is found that the lowest order transition (E1) on quartz substrate does not reduce to zero in the metallic phase as expected. There are two reasons: one is the slight decrease in the filled a1g bands compared to the bands due to a greater compressive stress in the quartz films; the other is associated with the donor level (ED) and acceptor level (EA).