Lattice strain in self‐ion‐implanted Si is investigated. Under certain irradiation conditions, a unique strain field is shown to form over the range of the ions. This strain field is one‐dimensional and expands the lattice along the direction normal to the surface of the crystal. This phenomena is investigated over a wide range of ion energy (100 keV and 1.25 MeV) and at different implantation temperatures. The presence of uniaxial strain is shown to correlate with a particular damage morphology in Si. This ion‐induced morphology and the irradiation conditions under which it forms are discussed, as well as the mechanism which leads to uniaxial, lattice strain. Both ion channeling and x‐ray diffraction analyses are used to characterize the strain field.