Surface morphology change of GaAs0.6P0.4 and GaAs layer grown on GaP substrate during growth was investigated by an Atomic Force Microscope. The compressive strain in the GaAsP layer was relaxed by {111} plane slips while that in the GaAs layer was relaxed by formation of island structures. During growth, the GaAs layer showed much flatter surface morphology. It was found that the mechanism of the strain relaxation largely affects the surface morphology. Finally, we compared the critical thickness for the {111} plane slip and the transition thickness in which the layer growth mode transits from a layer growth to an island growth to explain the different strain relaxation mechanisms. (C) 2012 Elsevier B.V. All rights reserved.