We study the growth of GaN crystal on Si substrate by molecular beam epitaxy (MBE), in order to integrate GaN light source and MEMS monolithically. Since the lattice constant of HfN is close to that of GaN (only 0.35% mismatch), the crystal growth of GaN on HfN film is superior. On the other hand, HfO 2 film is a good candidate for waveguide, dielectric and sacrificial layer. In this study, HfO2 film is surface-nitrified by a rf nitrogen plasma source of MBE to generate HfN layer. The morphology of the grown GaN crystal was better on the nitrified HfO2 layer. The photoluminescence (PL) efficiency of GaN quantum well grown on the nitrified HfO2 layer was better than that on Si substrate. As a simple hybrid light device structure, GaN grating on Si substrate was fabricated and the PL intensity from GaN diffraction grating was measured.