Electric fields are generally known to favor the ferroelectric polar state over the antiferroelectric nonpolar state for their Coulomb interactions with dipoles in the crystal. In this paper, we directly image an electric-fieldassisted ferroelectric-to-antiferroelectric phase transition during polarization reversal of the ferroelectric phase in polycrystalline Pb0.99{Nb0.02[(Zr0.57Sn0.43)0.92Ti0.08]0.98}O3.With the electric-field in situ transmission electron microscopy technique, such an unlikely phenomenon is verified to occur by both domain morphology change and electron-diffraction analysis. The slower kinetics of the phase transition, compared with ferroelectric polarization reversal, is suggested to contribute to this unusual behavior.