  The modification of the rotating vector model in the case of magnetars are
calculated. Magnetars may have twisted magnetic field compared with normal
pulsars. The polarization position angle of magnetars will change in the case
of a twisted magnetic field. For a twisted dipole field, we found that the
position angle will change both vertically and horizontally. During the
untwisting process of the magnetar magnetosphere, the modifications of the
position angle will evolve with time monotonously. This may explain the
evolution of the position angle in magnetar PSR J1622-4950 and XTE J1810-197.
The relation between the emission point and the line of sight will also change.
We suggest every magnetospheric models of magnetars also calculate the
corresponding changes of position angle in their models. Order of magnitude
estimation formula for doing this is given. This opens the possibility to
extract the magnetic field geometry of magnetars from their radio polarization
observations.
