Electrophilic C-H borylation of arenes using boron triiodide has been developed. This reaction proceeded smoothly in the absence of additives, and the diiodoboryl group was installed at the most sterically accessible carbon, where the HOMO is localized to a certain extent. Moreover, regioselective multiple borylation of polycyclic aromatic compounds was achieved by using excess boron triiodide. The borylated intermediates were transformed into a variety of arylboron compounds such as arylboronates, boronic acids, and trifluoroborates.