For the first time, the growth of ZnO nanobelts was monitored in situ using x-ray diffraction. The growth was carried out by heating metallic zinc powder in air at temperatures ranging from 368 to 568 °C. The morphology depends on both the growth temperature and the rate of heating to that temperature. A morphology diagram for the synthesized products was generated after systematic study of the experimental parameters. Higher temperatures and faster heating rates favor one-dimensional growth. Faster growth was observed for samples with higher growth temperatures, lower heating rates, and one-dimensional growth. These results give insight into the mechanism for the growth of ZnO nanobelts by metal oxidation.