Barium oxide (BaO) nanowires have been synthesized for the first time by using the composite hydroxide mediated (CHM) method. The products were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and transmission electron microscopy (TEM). Humidity sensors based on BaO nanowires were fabricated. The responsiveness to humidity for static and dynamic testing proved the ultrasensitive properties of the sensors. The resistance changed from 386 MΩ to 7.1 MΩ as the relative humidity (RH) increases from 20% to 95%. The response-time and recovery-time of the resistance is 16 s and 56 s versus the changes of relative humidity from 25% to 85%. These results indicate promising applications of BaO nanowires in a highly sensitive environmental monitoring and humidity controlled electronic device.