The 3D domain structure in ferroelectric Bi4Ti3O12 crystals is investigated by using piezoresponse force microscopy. Energetically favorable 90 degrees domain walls with faceted structure are established on the surface along the a-b plane, while the 90 degrees domain walls have complicated and curved surfaces along the c-axis inside the crystals (see figure). This domain structure can be explained by the strong interaction between the 90 degrees domain walls and oxygen vacancies.