We have applied low‐energy ion scattering to study the initial stage of InAs epilayer growth on GaAs by molecular‐beam epitaxy. We have first observed a characteristic variation of the scattered He intensity with respect to the incident angles of primary He+ions to the substrate surface. Then we have found a transition stage from the strained structure to the relaxed structure for the 4–15 monolayer thick InAs layer grown on GaAs substrate. Employing the method of surface shadowing for analysis of surface atomic steps, we have been able to observe the surface steps formation due to the island growth. We have discussed the correlation between the transition of growth process and the surface flatness from the standpoint of surface morphology.