  We calculate the polarization of radiation from thick accretion disks with
vertically averaged global magnetic field. The polarization arises as a result
of the radiation scattering by free electrons in magnetized plasma of a disk.
We consider as a basic effect the Faraday rotation of polarization plane along
the photon propagation in a magnetized disk. The various models of optically
thick accretion disk with a vertically averaged magnetic field are considered.
The main goal of this paper is to obtain simple asymptotic formulae for the
polarization of radiation when the Faraday rotation angle >> 1 at the Thomson
optical length ~1. The results of our calculation allows us to estimate the
magnetic field magnitude near the marginally stable orbit region of a black
hole via the data of polarimetric observations including the expected future
X-ray polarimetric observations. The wavelength dependence of polarization is
strongly dependent on various models of an accretion disks and thus allows to
choose a real model from the polarization data.
