  We study alignment of grains by radiative torques. We found steep rise of
radiative torque efficiency as grain size increases. This allows larger grains
that are known to exist within molecular clouds to be aligned by the attenuated
and reddened interstellar radiation field. In particular, we found that, even
deep inside giant molecular clouds, e.g. at optical depths corresponding to
less than Av of 10 large grains can still be aligned by radiative torque. This
means that, contrary to earlier claims, far-infrared/submillimeter polarimetry
provides a reliable tool to study magnetic fields of pre-stellar cores. Our
results show that the grain size distribution is important for determining the
relation between the degree of polarization and intensity.
