Due to the weak van der Waals interaction between GaN epitaxial layer and graphene substrate, GaN grown on graphene has attracted considerable attention in recent years, benefited from the possibility to grow epitaxial material without any necessity to satisfy the requirement for the lattice matching between the epitaxial materials and underlying materials, and the unique facility of transferring GaN epitaxy to other substrates. However, clusters formed in GaN grown on graphene lead to poor crystalline quality, deteriorating the applications of GaN epilayer on graphene. It is observed that preferential nucleation occurs primarily at the sites of defects and along the step edges of graphene. In order to study the effects of NH3/H2 ratio on the graphene/sapphire template and properties of GaN epilayer, the growth of GaN by metal organic chemical vapor deposition on the graphene/sapphire template pretreated with the mixed gas of NH3 and H2 is investigated.Prior to the deposition of GaN, five samples with different NH3/H2 flow ratios (0, 0.2, 0.5, 1 and 2, respectively) are pretreated at 1030℃ while the H2 flow rate is fixed at 3.6 mol/min. The surface topographies and Raman spectra of the pretreated graphene are investigated, and the chemical reaction mechanism is studied. It is found that the graphene is etched at the wrinkle firstly and then along the direction of wrinkles where there is bigger contact interface with NH3 and H2, and graphene decomposition is enhanced with the increase of NH3/H2 flow ratio. The pretreatment mechanisms of different mixed gases are also discussed. Owing to the weak bond energy, NH3 is easier to decompose than H2. The reaction between graphene and H, NH2 which are produced by the decomposition of NH3, enhances the etching of graphene.Finally GaN film is deposited on graphene/sapphire template pretreated by different NH3/H2 flow ratios. The quality of GaN was improved on graphene pretreated by appropriate NH3/H2 flow ratio and verified through highresolution X-ray diffraction.The lowest (002) and (102) full widths at half maximum (FWHM) of GaN obtained on graphene/sapphire template are 587 arcsec and 707 arcsec respectively, while the root-mean-square (RMS) of GaN is 0.37 nm. The stress of GaN is characterized by Raman spectra at room temperature. The co-presence of characteristic peaks of sapphire, graphene and GaN suggests that GaN has deposited on graphene/sapphire template. The E2-high Raman peak is used to estimate the residual stress in GaN material as described elsewhere. The E2-high peak of GaN grown on graphene is around 566.7 cm-1, while the value of strain-free GaN is 566.2 cm-1. Thus, there is subtle compressive stress in the GaN grown on graphene, which can be calculated from the relationship:△ωγ=4.3·σχχ cm-1·GPa-1, giving a value of 0.11 GPa of GaN obtained on graphene/sapphire template.This study provides an effective pretreatment technique to improve the crystal quality of GaN epilayer deposited on graphene/sapphire template, which gives guidance in well crystallizing three-dimensional materials on two-dimensional materials.