The lateral distribution of damage introduced in semiconductors by ion implantation is studied theoretically using the theory of Lindhardetal. This theoretical study is experimentally verified by a backscattering measurement of the damage formed by ion implantation into a tilted target. It is found that the spread of lateral damage is largest at a position near to or deeper than the ion projected range, and that the ratio of the lateral damage spread from a mask edge to the longitudinal damage spread from the surface is less than unity even in the case of implantation of light ions. For instance, the ratio is about 20% in the case of 100‐keV B+implantation into Si. It is also found that the ratio becomes smaller as the energy of the implanted ions increases or as the ions become heavier.