The behavior of monolayer-deep holes on the surface of (001) GaAs during post-growth annealing in molecular beam epitaxy was observed by in-situ scanning electron microscopy. Most small holes disappear immediately after growth. However, it was found that some are left and combine to form big holes. They extend in the [110] direction and coalesce with each other, while at the same time, shrink in the [110] direction. Finally, they shrink in both directions and disappear. It takes about 10min for all the holes to disappear, which is much longer than the growth interruption period usually employed to smooth heterointerfaces. The anisotropic behavior of big holes is discussed in relation to the reported growth anisotropy.