Nanoparticles of NiO (NP-NiO) were prepared by a novel sonochemical route from Ni acetate and sodium hydroxide without any requirement of calcinations steps at high temperature and without surfactants. Drop casting of the Nano crystals onto alumina substrates allowed the fabrication of gas sensing devices, which were tested towards NO2 and CO and showed promising results. At low working temperature, the NiO nanoparticles based sensors are selective to nitrogen oxide; in fact a good sensitivity is shown at 200◦ C at low concentration (2 ppm), while at temperature above 350◦ C, high responses are obtained for carbon monoxide. The results obtained are stimulating for further developing of nano-NiO based sensor devices.