  We investigate the structure of the Mg II broad line emission region in a
sample of 284 Type 1 active galactic nuclei (AGNs), through comparing the
kinematical parameters of the broad Mg II and broad H$\beta$ lines. We found
that the Mg II emitting region has more complex kinematics than the H$\beta$
one. It seems that the \ion{Mg}{ii} broad line originates from two subregions:
one which contributes to the line core, which is probably virialized, and the
other, 'fountain-like' emitting region, with outflows-inflows nearly orthogonal
to the disc, which become suppressed with stronger gravitational influence.
This subregion mostly contributes to the emission of the Mg II broad line
wings. The kinematics of the Mg II core emitting region is similar to that of
the H$\beta$ broad line region (seems to be virialized), and therefore the Full
Width at Half Maximum (FWHM) of Mg II still can be used for the black hole (BH)
mass estimation in the case where the Mg II core component is dominant.
However, one should be careful with using the \ion{Mg}{ii} broad line for the
BH mass estimation in the case of very large widths (FWHM$>$ 6000 km s$^{-1}$)
and/or in the case of strong blue asymmetry.
