Five types of ferroelectric domain walls (DWs) are present in Bi4Ti3O12 single crystals (Ye et al., 2015). Here their motion was investigated in situ using transmission electron microscopy and optical microscopy. The motion of P
(a)-90° DWs, P
(a)-180° DWs and P
(c)-180° DWs was observed through electron beam poling in a transmission electron microscope. The growth of new P
s(a)-180° nanodomains was frequently seen and they tended to nucleate at preexisting P
s(a)-90° DWs. Irregularly curved P
(c)-180° DWs exhibit the highest mobility, while migration over a short range occurs occasionally for faceted P
s(a)-90° DWs. In addition, the motion of P
s(a)-90° DWs and the growth/annihilation of new needle-like P
s(a)-90° domains in a 20 µm-thick crystal were observed under an external electric field on an optical microscope. Most of the new needle-like P
s(a)-90° domains nucleate at preexisting P
s(a)-90° DWs and the former are much smaller than the latter. This is very similar to the situation for P
s(a)-180° domain switching induced by electron beam poling in a transmission electron microscope. Our observations suggest the energy hierarchy for different domains of P
s(c)-180° ≤ P
s(a)-180° ≤ P
s(a)-90° ≤ new needle-like P
s(a)-90° in ferroelectric Bi4Ti3O12.
