Pure and Zn-doped SnO2 nanocrystals were prepared through a facile solid-state reaction, in which zinc doping can induce the generation of orthorhombic SnO2 and control the crystallite size of nanocrystals. Most Zn-doped SnO2 nanocrystals showed tetragonal rutile structure as well as a few orthorhombic structures. With an increase in the molar ratio of Zn/Sn from 0 to 0.133, the content of orthorhombic structure in the product gradually increased and the crystallite size of nanocrystals decreased, accompanied by an increase in sensor response of the nanocrystals to acetone. When the molar ratio of Zn/Sn was 0.133, the obtained nanocrystals had minimum crystallite size of 9.7 nm and showed the highest response of 48–100 ppm acetone, which is four times larger than that of undoped SnO2. With increasing the molar ratio of Zn/Sn to 0.4, a small amount of hexagonal ZnO was produced and the crystallite size increased although more orthorhombic structures were formed, resulting in a decrease of sensor response. Furthermore, the effects of crystallite size and the content of orthorhombic SnO2 on the gas-sensing properties of Zn-doped SnO2 nanocrystals were also discussed in detail.