SnO 2 nanospheres with a diameter of about 150 nm were synthesized by a hydrothermal method. They were assembled by numerous tetragonal nanocubes with an average size of 8 nm. Since the morphology of nanospheres agglomerated by nanocubes, the modulations of quantum confinement and large contact area were realized; this would serve as the basis for high sensitivity. The sensors based on the as-synthesized nanospheres showed good selectivity among different gases and superior sensitivity to ethanol compare with previous reports. The sensitivity could reach 53 to 50 ppm ethanol at a relatively low working temperature of 275 °C. A possible mechanism was proposed, which could be primarily related to the large surface-to-volume ratio and the small size effect of nanocubes.