The significant dependence of strain state of lattice of GaAs films grown by molecular-beam epytaxy technique (MBE) on the nucleation method of the first layers of GaP buffer (50 nm) on vicinal substrate Si(001) 4° round <;011> was revealed. GaP growth started layer-by-layer with gallium or phosphorus sublayer. In the case of GaP nucleating with gallium, GaAs film has significant lattice rotation round <;011>. When buffer starts forming with phosphorus layer GaAs film is evident to rotate round <;001>. Film relaxation degree exceeds 100%, it is in the lateral strained state. The analysis was carried out using the triclinic distortion model. The reciprocal scattering map obtained using X-ray diffraction in the three-axis small enabling circuit is presented. The map evidently shows that GaAs film lattice is rotated.