Initial stage of lattice relaxation of GaAs islands grown on Si(100) substrate were investigated by combination of reflection high-energy electron diffraction and molecular beam epitaxy. In addition to the lattice constants in horizontal direction(a∥)to the substrate surface, we first measured directly those in vertical one(a⊥).At the beginning of the growth, the rapid increase ofa∥and the larger Poisson’s ratios than that of bulk were observed. Atomic bond flexibility and extension induced by surface effects might cause this rapid increase ofa∥and large Poisson’s ratios. ©1997 American Institute of Physics.