SnO 2 hollow nanospheres have been synthesized by heat treatment of a mixture composed of SnCl 4 and carbon microspheres. The SnO 2 nanospheres are 100 nm in size and composed of large amount of SnO 2 nanocrystals with diameters of ~6 nm, which is close to two times of the Debye length of SnO 2 . It was found that the sensitivity to 1000 ppm ethanol obtained with SnO 2 hollow nanosphere-based sensors is as high as 75. The response and recovery time are only 4 and 10 s, respectively. Such excellent sensing properties can be ascribed to the small-size nanocrystals and porous structure of SnO 2 hollow nanospheres, which lead to large diffusion constant.