As an earth-abundant narrow bandgap semiconductor material with mature manufacturing process, silicon has already been extensively used in photovoltaic industry. However, its low stability in aqueous conditions becomes an obstacle for its application in artificial photosynthesis. Thus, a great amount of efforts has been directed towards developing protection layer for Si-based photoelectrode. Deposition of metal oxides, such as TiO₂, on Si have been reported to enhance its long-term stability. However, crystallinity and thickness of these metal oxides often limit the performance of Si-based photoelectrodes. On the contrary, MBE-grown GaN possesses extraordinarily chemical, thermal, and mechanical stabilities with low defect density and high crystallinity. These characteristics make GaN an appropriate candidate for Si photocathode protection. Our study shows that, the MBE-grown GaN on p-type Si can well protect the Si photocathode for more than 100 hrs. Various thicknesses of GaN were examined and showed similar photoelectrochemical performance. Our study reveals that the band alignment between Si and GaN ensures similarly efficient electron extraction at different GaN thicknesses. Interestingly, this behavior is in stark contrast to the well-known TiO₂ protection coating. A self-healing effect was also observed during chronoamperometry test and the surface was characterized afterwards by XPS, pc-AFM and TEM. The results indicate that the surface was modified during solar water splitting, leading to more efficient charge carrier extraction and more stable surface.