P-type conducting undoped tin oxide thin film gas sensor is fabricated by direct current reactive magnetron sputtering method and calcination technique. Physical characteristics of the undoped tin oxide thin films have been analyzed by Spectroscopic ellipsometer, x-ray diffraction (XRD), scanning electronic microscope, and atomic force microscopy. According to the ethanol sensitivity properties of the sensors, we find that the calcination temperature and the thickness of the films are correlated with the special p-type conducting type; the p-type ultrathin film (10 nm) gas sensor shows better gas sensitivity and less baseline shift. XRD studies indicate that the preferred unidentified diffraction peak at 33.082° favors the formation of p-type conducting. When the intensities of unidentified diffraction peak increases, the gas sensing properties is largely promoted. The response time of the p-type sensor is less than 1 s to 1000 ppm ethanol.