Theoretical and experimental X-ray diffraction investigations of the lattice strain caused in the surface layer of Si single crystals by monoenergetic Au ions (ion energy 120 and 180 keV, doses 1013 and 2 x 1014 ions/cm2, respectively) through implantation are carried out. The traverse-type topography by Lang's method is used. The X-ray radiation absorption and the polarization are taken into account.