MOSFETs on GaN are one of the most critical issues for realizing power applications. We have found the mo-bility is considerably enhanced on the specially treated GaN surface by high temperature annealing after AlN deposition. The surface structure was analyzed by TEM and XPS. The field-effect mobility strongly depends on the Al concentration of the surface. The mechanism is still unclear, however, the significant enhancement of channel mobility is expected for realization of high performance vertical GaN MOSFETs.