ZnO nanowires were grown on 2-μm-thick GaN templates by chemical vapor deposition without employing any metal catalysts. The GaN template was deposited by metal-organic chemical vapor deposition on a c-plane sapphire substrate. The diameters of the resulting nanowires were in the range of 40–250nm depending on growth time. The ZnO nanowires were vertically well aligned with uniform length, diameter, and distribution density as revealed by electron microscopy. X-ray diffraction spectra showed that ZnO grew in single c-axis orientation with the c axis normal to the GaN basal plane, indicating a heteroepitaxial relationship of (0002)ZnO‖(0002)GaN. The lattice constant of the c axis of the ZnO nanowires with diameter of 40nm was 5.211Å, which is larger than that of bulk ZnO (5.207Å). The ZnO nanowires exhibit a residual tensile strain along the c axis, which decreases with increasing diameter.