Annealing behavior of structure, nature, and two‐dimensional depth distributions of secondary defects in one‐line‐scan, 70‐keV focused ion beam B‐implanted (100) Si have been investigated mainly using cross‐sectional transmission electron microscopy observations. In as‐implanted layers for doses above 3×1015/cm2, triangular amorphous regions are formed with a two‐dimensional spread on the order of a 0.2 μm, which is approximately an ion beam diameter. Secondary defects generated by annealing above 800 °C are confined within these triangular regions. By contrast, when 8×1014/cm2implanted layers are annealed at 1000 °C, two‐dimensional spreads of the grown defects vary from 0.2 to 0.6 μm in each implanted layer. However, in annealing at 800 °C, these defects remain near the projected range of boron and their lateral spread is confined to a beam diameter. Some results are discussed in comparison with two‐dimensional impurity distributions, 140‐keV P++implantation, and Monte Carlo simulations.