Nano-sized nickel oxides have been synthesized in a water-in-oil microemulsion. The as-synthesized samples were characterized by powder X-ray diffraction (XRD), transmission electronic microscopy (TEM) and nitrogen adsorption/desorption. The particle size of nickel oxide can be controlled from 11.5 to 31.5 nm by varying the proportion of water, surfactant and oil in the microemulsion, mixing method, and calcining temperature. Gas sensors based on as-synthesized nickel oxide are fabricated and investigated. They exhibit much higher sensitivities to hydrogen sulfide, ethanol and nitrogen dioxide, compared to those based on the conventional bulk NiO. Furthermore, the response of as-synthesized materials to various kinds of target gases increases with the decreasing of the particle size of gas sensors. It is noted that the NiO sensor with particle size of 11.5 nm displays high degree of selectivity, coupled with high response value, making it particularly interesting for H2S-monitoring applications.