We investigated the crystal structure and ferroelectric domains of Bi₄Ti₃O₁₂ by the single-crystal X-ray diffraction and transmission electron microscopy (TEM) techniques. The space group in the ferroelectric phase was found to be P1a1. We successfully refined the crystal structure based on this space group. The calculated spontaneous polarization by the point charge model is good agreement with the reported experimental value. The 180° and 90° ferroelectric domain structures were observed by the [001]-zone dark-field TEM imaging. In the 180° domain structure, we determined that one component of the polarization vector is along the a-axis. The annular bright-field scanning transmission electron microscopy images displayed every atomic position in spite of the highly distorted structure of Bi₄Ti₃O₁₂. The tilting and distortion of the TiO₆ octahedra were also clearly observed. Therefore, we directly evaluated the ferroelectric displacements of Bi and Ti ions.