Lattice strain of Si(111) implanted 1.5 MeV Au²⁺ ion was investigated by extremely asymmetric x-ray diffraction. The measured rocking curves were consisted of a bulk peak and a broad sub peak accompanying with intensity oscillation. Analysis of the strain distribution was done by fitting of the measured curve with curves calculated by a dynamical diffraction theory. The resultant strain profile shows introduction of a tensile strain in extent of 500 nm. Comparing the strain profiles with the distributions of the projected range and the vacancy calculated by the TRIM code, it was concluded that the lattice strain is contributed by not only the defect due to collision event but also the interstitials of Au ion. In addition, we found that the global shape of the broad sub peak is very sensitive to the strain distribution within the depth of ∼80 nm. [DOI: 10.1380/ejssnt.2006.25]