  The anisotropy and polarization of the cosmic microwave background radiation
(CMBR) induced by the scalar and tensor metric perturbations are computed in
the long-wavelength limit. It is found that the large-scale polarization of
CMBR induced by the decaying tensor mode can reach a few percents. This is
different from the scalar or inflation-induced tensor mode, whereas the
polarization is at least two orders of magnitude lower. The effect of matter
re-ionization on CMBR is also considered. We conclude that measuring the
polarization of CMBR on large-angular scales can probe the ionization history
of the early Universe, and test different cosmological models.
