A green and low-cost method to prepare high-quality GaN (gallium nitride) nanowires is important for the applications of GaN-based devices on a large scale. In this work, high-quality GaN nanowires are successfully prepared by a green plasma enhanced chemical vapor deposition method without catalyst, with Al₂O₃ used as a substrate, metal Ga as a gallium source and N₂ as a nitrogen source. The obtained GaN nanomaterials are investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, and photoluminescence (PL) spectroscopy. The XRD results demonstrate that hexagonal-wurtzite GaN is obtained and no other phases exist. The SEM results show that GaN nanowires and hexagonal GaN microsheets are obtained at different temperatures. When the growth temperature is at 950 ℃ (reaction time for 2 h), the hexagonal GaN microsheets each with a size of 15 μm are obtained. When the growth temperature is at 1000 ℃(reaction time for 2 h), the GaN nanowires with the lengths in a range of 10–20 μm are obtained. With the reaction temperature increasing from 0.5 h to 2 h, the lengths of GaN nanowires increase. The TEM results suggest that the GaN nanowires are of high crystallinity and the growth direction of GaN nanowires is in the 0001 direction. The Raman results indicate that there exists a compressive stress in the GaN nanowires and its value is 0.84 GPa. Meanwhile, the growth mechanism of GaN nanowires is also proposed. The morphologies of GaN nanomaterials are tailed by the growth temperature, which may be caused by Ga atomic surface diffusion. Ga atoms have low diffusion energy and small diffusion length at 950 ℃. They gather in the non-polar m-plane. The (0001) plane with the lowest energy begins to grow. Then, hexagonal GaN microsheets are obtained. When reaction temperature is at 1000 ℃, the diffusion length of Ga atoms increases. Ga atoms can diffuse into (0001) plane. In order to maintain the lowest surface energy, the GaN nanowires grow along the 0001 direction. The PL results indicate that the obtained GaN nanowires have just an intrinsic and sharp luminescence peak at 360 nm, which possesses promising applications in photoelectric devices such as ultraviolet laser emitter. Our research will also provide a low-cost and green technical method of fabricating the new photoelectric devices.