The behavior of monolayer holes on GaAs(001) surface during post-growth annealing in molecular beam epitaxy (MBE) is examined in detail by in-situ scanning electron microscopy (SEM). Submicron scale, nearly rectangular, monolayer deep holes are formed after small islands and holes are eliminated. Their growth and shrinkage are found to proceed asymmetrically: For example, they grow only into the right side, and shrink only from the top. The mechanism is discussed in terms of step activity. It was found that in the regrowth after annealing, three dimensional islands are formed preferentially on the step edges.