The room temperature gas sensors have always been an important research direction of the gas sensor, and the room temperature gas sensors without the assistance of the light is more valuable. The SnO 2 nanoparticles were synthesized by hydrothermal method, which showed good formaldehyde sensitivity, had the advantages of low test temperature, only [Formula: see text]C, good formaldehyde selectivity, and especially the good response to formaldehyde at room temperature. The nanostructure and gas-sensing properties were characterized by scanning electron microscopy, EDS mapping, nitrogen physical adsorption, and X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and WS-60B gas-sensing measurement system. Compared with the reported research results, we carefully discuss the physical mechanism of the SnO 2 formaldehyde sensor with low operating temperature and good formaldehyde selectivity in this paper.