Structural studies on highly conductive thin films of pentacene (PEN) were carried out by x‐ray and electron diffraction methods. The ultrathin film before and after doping with iodine was confirmed to consist of single crystalline domains, and the lattice parameters were determined. Drastic elongation along thecaxis as a result of doping was observed, while thea‐bplane did not change during doping. The intercalation of iodine molecules between molecular layers of PEN was confirmed.