Strong c-axis-oriented hexagonal (0001) GaN was grown on graphene/Si(100) substrates by radio-frequency plasma-excited molecular beam epitaxy. The hexagonal symmetry of graphene transferred onto the Si(100) surface enabled the growth of a highly c-axis-oriented GaN film. The GaN showed a full width at half maximum of 11.3 arcmin for a (0002) rocking curve measured by X-ray diffraction. Strong luminescence at 3.4 eV was also observed by cathodoluminescence with a luminescence at 3.2 eV, which originated from a cubic-phase inclusion. A microstructural study using transmission electron microscopy also confirmed the growth of hexagonal (0001) GaN on a graphene/Si(100) substrate. (C) 2014 The Japan Society of Applied Physics