I estimated anisotropy of stress and strain for GaAs heteroepitaxial films grown on vicinal Si(001) and Si(110) substrates by molecular-beam epitaxy from measurements of in-plane substrate curvatures and components of lattice parameters by a Bond method using x-ray diffraction. It is shown that the GaAs heteroepitaxial film is anisotropically strained through different relaxation processes of stress due to anisotropic dislocations along the [110] and [1 (1) over bar0] directions. The crystal lattice of the GaAs heteroepitaxial film on Si substrate is qualitatively represented considering the crystal structure elastically strained with the [110]-, [1 (1) over tilde0]-, and [001]-primitive axes as a primitive basis in the orthogonal rhombic system.