  We discuss the contribution of gravitational wave to the cosmic microwave
background radiation (CMBR) anisotropy and polarization. It is found that the
large-scale polarization of CMBR is less than 1\% for a standard recombination
universe. The effect of matter reionization will enhance the CMBR polarization
to a 10\% level. We have computed the CMBR polarization for two extreme cases
(not absolutely ruled out) and found that further enhancement of the ratio is
possible. We conclude that measuring the polarization of CMBR on large-angular
scales can probe the ionization history of the early universe, set constraints
on baryon density and the spectral index of the gravitational waves.
