Properties of unintentionally-doped GaN re-grown on molecular beam epitaxy grown AlN/Sapphire templates by metal organic vapor phase epitaxy (MOVPE) are studied in this article. X-ray diffraction (XRD), transmission electron microscope (TEM), and atomic force microscope are used to investigate the influence of the crystal quality and surface morphology of AlN on the GaN. It is found that when surface roughness of AlN is small, the GaN has a full width at half maximum (FWHM) values of XRD rocking curves (200—300 and 400—500 arcsec for (002) and (102) plane ω-scan, respectively) and surface roughness (0.1—0.2 nm), which are comparable to those grown on sapphire substrates by using "two-step" method, although the FWHMs of (102) plane XRD ω-scan curves of AlN are 900—1500 arcsec. The reason for dislocation reduction in GaN shown by TEM image is that a part of dislocations in AlN are eliminated in the interface between AlN and GaN. This is probably due to the lattice restoration from Ga atoms for their large size. On the other hand, when surface roughness of AlN is large, the surface migration of Ga atoms is nestricted during the MOVPE growth, which results in a poor GaN quality. Moreover, the resistivity of GaN confirmed with Van der Pauw method is between 105 and 106 Ω ·cm, which is about six orders of magnitude higher than that in GaN grown on sapphire substrates. This is attributed to the replacement of low temperature GaN buffer layer by the AlN.