The lateral spread of implanted ions is measured in boron‐implanted silicon by junction delineation. The maximum lateral spread of ap‐type region from a mask edge is about half the junction depth. Typical values are 0.4–0.6 &mgr; inn‐type substrates of 0.1 &OHgr; cm at an implantation energy in the range 75–250 keV with the dose level of 1×1015/cm2, or at 150 keV with the dose in the range 1×1014–5×1015/cm2. These experimental values are well interpreted by the theory in which the lateral spread is expressed as a complementary error function with the standard deviation calculated by the LSS theory.