Humidity dependence of gas sensors is one of the main obstacles for practical application. Through Ru doping into NiO flower-like microspheres, which are synthesized by a one-step hydrothermal route, the gas sensors exhibit high stability to humidity changes. The sensors based on 0.5 at% Ru-doped NiO have almost the same responses to 100 ppm acetone at 200℃when the relative humidity ranges from 15 % to 90 %. At the same time, they possess the highest response (similar to 12 to 100 ppm acetone), which is about 9 times higher than that based on pure NiO. In addition, the sensors based on 0.5 at% Ru doped NiO sample show good selectivity and good long-term stability. Gas sensing mechanism for the enhanced gas sensing performance is also discussed. The results demonstrate that doping Ru into NiO flower-like microspheres is a promising way for designing high performance acetone gas sensors.