Zn wire was used as starting material in the fabrication of ZnO crystals in which a glass substrate was placed above the wire to grow ZnO crystals. The wire was heated by electric current in air. When the wire broke because of Joule heating, smoke arose from the wire and ZnO crystals were observed on the broken point, in the vicinity of the broken point, and on the glass substrate. The morphology and cathodoluminescence of the crystals were investigated. The crystals on the wire were 0.2–5 μm in size. The peak intensity ratio of ultraviolet (UV) emission to green emission increased with decreasing crystal size. The crystals on the substrate were tetrapod-like; the length and diameter of the tetrapod legs were 100–500 nm and 10–30 nm, respectively. The tetrapod-like nanocrystals produced only UV emissions.