The island-formation processes of GaAs heteroepitaxial growth on InP (lattice constant ( a) is 3.7% larger than that of GaAs) and InAs ( a is 7.2% larger than that of GaAs) surfaces were investigated using a scanning tunneling microscope (STM) multichamber system equipped with a molecular beam epitaxy facility. In the case of GaAs/InP heteroepitaxial growth, two-dimensional (2D) growth was observed when less than 2.0 monolayers (MLs) of GaAs were deposited on the InP surfaces. On the other hand, three-dimensional (3D) growth was confirmed for more than 2.0 ML GaAs deposition. In the case of GaAs/InAs heteroepitaxial growth, the same transition from 2D to 3D island growth occurred when more than 0.75 ML of GaAs was deposited.