The structural transformation in self‐ion‐implanted Si has been investigated using Raman spectroscopy and Rutherford backscattering spectrometry. The crystal Si Raman peak at 520 cm−1decreased, broadened, and shifted toward lower wave numbers as the 100 keV Si+dose was increased from 0.8 to 8.5×1014cm−2. These peak shifts can be attributed to uniaxial lattice expansion in the direction normal to the Si surface and they are substantially larger than those predicted in uniaxially strained crystal Si. The results suggest that accumulated defects generated by ion bombardment not only expand the crystal Si lattice but also reduce the force constant which in turn increases plasticity and finally gives rise to amorphization of the Si lattice.