  We investigate the effect of gravitational lensing by matter distribution in
the universe on the cosmic microwave background (CMB) polarization power
spectra and temperature-polarization cross-correlation spectrum. As in the case
of temperature spectrum gravitational lensing leads to smoothing of narrow
features and enhancement of power on the damping tail of the power spectrum.
Because acoustic peaks in polarization spectra are narrower than in the
temperature spectrum the smoothing effect is significantly larger and can reach
up to 10\% for $l<1000$ and even more above that. A qualitatively new feature
is the generation of $B$ type polarization even when only $E$ is intrinsically
present, such as in the case of pure scalar perturbations. This may be directly
observed with Planck and other future small scale polarization experiments. The
gravitational lensing effect is incorporated in the new version (2.4) of
CMBFAST code.
