The preparation and electrical properties of a sintered body consisting of cubic spinel oxide MnFeNiO4 were investigated. A sintered body with cubic spinel and NaCl-type oxides prepared at 1400 degrees C in Ar could be converted to one consisting of monophase cubic spinel oxide by oxidation at 1000 degrees C for more than 48 h in air. The electrical conductivity of a sintered body consisting of monophase cubic spinel oxide was confirmed to increase exponentially with increasing temperature, indicating that the oxide has intrinsic negative temperature coefficient (NTC) thermistor characteristics. The electrical conduction of the oxide was concluded to be controlled by the small polaron hopping mechanism. Changes in electrical conductivity, mobility, and charge carrier jump frequency in temperature dependence at 400 degrees C are assumed to be related to the variation in cation distribution accompanying the disproportionation of Mn ions.