As the third generation semiconductor material, gallium nitride (GaN) is widely used in electronic devices and optoelectronic devices due to its excellent characteristics such as wide band gap, high breakdown field strength, high electron mobility, outstanding thermal conductivity, and direct band gap. However, it is difficult to obtain high quality single crystal GaN thin films due to the mismatch between GaN material and substrate in early phase of preparation. Until the two-step growth method is proposed, in which the nucleation layer of aluminum nitride (AlN) is firstly grown on the substrate at low temperature, and then GaN is grown at high temperature, the quality of GaN is greatly improved. Nowadays, AlN nucleation layers are fabricated via magnetron sputtering and molecular beam epitaxy, etc. To further improve the quality of GaN crystals, this study used plasma-enhanced atomic layer deposition (PEALD) method to prepare AlN nucleation layers for the epitaxial growth of GaN on a two-inch c-plane sapphire substrate. Compared with the magnetron sputtering method and molecular beam epitaxy method, the crystal quality of AlN prepared by PEALD method displays advantages of simple process, low cost and high yield. Measurements on deposited AlN films show that the deposition rate is 0.1 nm/cycle and the films have island-like structures varying with its thickness. Epitaxial GaN measurements show that GaN epitaxial layer can obtain the smoothest surface with a root mean square roughness of 0.272 nm, the best optical properties, and the lowest dislocation density when AlN is deposited with a thickness of 20.8 nm. In conclusion, a new method of epitaxial single crystal GaN on AlN prepared by PEALD has been built with optimal deposition at 20.8 nm of AlN to obtain high quality GaN thin films, it can be used to prepare high electron mobility transistors and light-emitting diodes.