  In previous investigations on zitterbewegung(zbw) of electron, it is believed
that the zbw results from some internal motion of electron. However, all the
analyses are made at relativistic quantum mechanical level. In framework of
quantum field theory (QFT), we find that the origin of zbw is different from
previous conclusion. Especially, some new interesting conclusions are derived
at this level: 1) the zbw arises from the rapid to-and-fro polarization of the
vacuum in the range of the Compton wavelength (divided by $4\pi$) of the
electron, which offer the four-dimensional(4D) spin and intrinsic
electromagnetic-moment tensor to the electron; 2) Any attempt that attributes
spin (rather than double the spin) of the electron to some kind of orbital
angular momentum would not be successful; 3) the macroscopic classical speed of
the Dirac vacuum medium vanish in all inertial systems.
