  We discuss the effect of matter reionization on the large-angular-scale
anisotropy and polarization of the cosmic microwave background radiation (CMBR)
in the standard CDM model. We separate three cases in which the anisotropy is
induced by pure scalar, pure tensor, and mixed metric perturbations
respectively. It is found that, if reionization occurs early enough, the
polarization can reach a detectable level of sequentially $6\%$, $9\%$, and
$6.5\%$ of the anisotropy. In general, a higher degree of polarization implies
a dominant contribution from the tensor mode or reionization at high redshift.
Since early reionization will suppress small-scale CMBR anisotropies and
polarizations significantly, measuring the polarization on few degree scales
can be a direct probe of the reionization history of the early universe.
