This is the first report of the observation of mass transport of single crystalline GaN. A wafer with the squared grooves was annealed at 1100 degrees C under flows of NH3 and N-2. During the annealing, no group-III alkyl source gas was supplied. After 12 min annealing, all the stripes were buried due to a mass transport of GaN from the unetched region. The mechanism of mass transport is discussed. The strong crystallographic anisotropy of GaN is expected to modulate the shape of the mass transported region. Transmission electron microscopy revealed that threading dislocations were bent and did not climb through the mass transported region. Therefore, a dislocation-free region was achieved at the upper part of the mass transported region except for one dislocation observed at the center of the groove. This method is promising for the fabrication of dislocation-fi ee GaN on sapphire substrate.