The SnO2 nanorods with a diameter down to 3nm are synthesized through a hydrothermal route and their sensing properties are investigated. The small size is confirmed by the results of x-ray diffraction, transmission electron microscope and Raman scattering spectrum. The sensitivity is up to 83.8 as the nanorod sensor is exposed to 300ppm ethanol vapor in air. Moreover, the linear dependence of the sensitivity on the ethanol concentration is observed for each of the 20 sensors. Compared with the measured results of 80–180nm tin oxide particles, such linear dependence is related to the small size effect.