Highly crystalline SnO₂ nanocrystals with and without Zn²⁺ doping were directly prepared by a solvothermal method. By systematic characterizations using x-ray diffraction, transmission electron microscopy, infrared spectra, and UV–vis spectra, it is demonstrated that all samples crystallized in a single phase of rutile structure, and that upon Zn²⁺ doping particle sizes closer to the exciton Bohr radius (2.7 nm) of SnO₂ were achieved, while the particle size of SnO₂ nanoparticles was as large as about 12 nm without Zn²⁺ doping. The smaller particle sizes for Zn²⁺ doped nanoparticles had led to a lattice expansion, a blue-shift of the surface phonon mode for the anti-symmetric Sn–O stretching vibration, and a significantly narrowed band-gap energy, opposite to what is theoretically predicted by the quantum size effect. The photocatalytic activity of the doped samples is measured by monitoring the degradation of methylene blue dye in an aqueous solution under UV-radiation exposure. It is found that Zn²⁺ doped SnO₂ showed excellent activity toward photodegradation of methylene blue solution under UV light irradiation. These observations were interpreted in terms of the Zn²⁺ doping at the surface sites of SnO₂ nanoparticles and the relevant defects that have increased the surface active sites and moreover improved the ratio of surface charge carrier transfer rate to the electron–hole recombination rate. These results illustrate the potential of rationally modifying the particle size and surface defect characteristics for novel semiconductor oxide photocatalysts.