We calculated the dynamics of indium atom during the molecular beam epitaxy growth of InAs on GaAs(0 0 1) using the first-principle calculation. The result shows us that indium atom is very mobile. The hopping barrier energy of In is almost same as that of As on GaAs(0 0 1) surface. The substitution of In toward the substrate GaAs(0 0 1) also occurred, because the energy difference between the In atom on the second topmost layer and the substitution of it with the Ga atom of the fourth layer is very small. Thus, we should be careful to analyze experiments because indium atom is very active and mobile on GaAs(0 0 1) surface. (C) 2003 Elsevier Science B.V. All rights reserved.