Novel ultra-fine NiO/SnO₂ nano-particles (NPs) have been fabricated by creatively using carbon black (CB) as a frame and dispersing agent and applied as gas-sensing materials. In a typical synthesis, a precursor composed of mixed NiO/SnO₂ NPs and CB was initially prepared. Subsequently, the ultra-fine NiO/SnO₂ NPs were obtained by a thermal treatment of the precursor. The final products were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, and Brunauer Emmett Teller N₂ adsorption–desorption analysis. The ethanol-sensing tests revealed that the ultra-fine NiO/SnO₂ NPs used as gas sensors have a quick response/recovery capability (response time: 2–40 s; recovery time: 3–20 s) within a detecting range of 5–100 ppm at an operating temperature of 300 °C; strikingly, the response/recovery times are quite short (2 s and 3 s, respectively) even under the detection limit of 5 ppm. As demonstrated, the ultra-fine NiO/SnO₂ NPs are highly promising for real-time monitoring gas sensor applications.