We examined dynamic behavior of field-emitting carbon nanotubes (CNTs) by in situ transmission electron microscopy (TEM). CNTs employed in the present study were multi-wall CNTs prepared by chemical vapor deposition, double-wall and single-wall CNTs produced by arc discharge. Orientation of CNTs, being random when no electric field was applied, were aligned parallel to the electric field and returned reversibly to their original direction when the field was turned off. In addition to this reversible behavior without serious structural damage in CNTs, sublimation and violent oscillation of CNTs were observed. When CNTs were bundled, branching of the bundle by electric static force was also observed.