The surface diffusion length of Ga adatom incorporation under As-2 and As-4 mixture flux was measured using microprobe-RHEED intensity oscillations in molecular beam epitaxy. The incorporation diffusion length of Ga on the (0 0 1) surface towards the [1 1 0] orientation showed a strong dependence on the As-2/As-4 ratio in the incident beam from the cracker cell. In keeping the total number of As atoms in the beam constant, it was found that the incorporation diffusion length of Ga decreases with higher As2As4 ratio. This shows that As, molecules are partly incorporated into the GaAs epi-layer by the dissociative reaction at Ga atoms, while As, is almost completely incorporated. According to the calculation for the diffusion length of Ga. a single As-4 molecule decomposes to produce one As-4 molecule which is incorporated into the growing layer, whereas another ASL molecule is lost by desorption. On the other hand. almost all As-2 molecules are active and adsorbed. This means that As-2 molecules are incorporated into GaAs epi-layer more efficiently than As-4 molecules.