Series of negative temperature coefficient Mn-Co-Ni-O spinel oxide thin films were successfully prepared on thermally oxidized SiO2/Si (100) substrates by RF sputtering a Mn-Co-Ni metal alloy target with the sputtering gas of different Ar:O-2 partial pressure ratios. The crystallinity of the spinel films was improved as O-2 contents increased. The film conductivity was reduced. The minute variation in an optical absorption peak at around 1.7 eV by spectroscopic ellipsometry may be ascribed to the different charge transfer mechanism between Co2+ in tetragonal and Co3+ in octahedral or between O2- and Mn4+ in octahedral sites. Raman and XPS results provided further evidences.