The hollow Co3O4 microspheres were prepared by a gas-liquid diffusion reaction in the presence of ionic liquid [Bmim] [BF4] in combination with calcination at 300 degrees C. Their structures and morphologies were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman spectrometry, and X-ray photoelectron spectroscopy. The growth mechanism of hollow Co3O4 microspheres was proposed. The ethanol sensing properties were measured using a WS-30A gas sensor measurement system. The influence of working temperatures, ethanol concentrations, and specific surface areas of Co3O4 microspheres on the ethanol sensing properties was investigated. The hollow Co3O4 microspheres showed excellent sensitivity to ethanol vapor at a lower operating temperature.