  We present the transport studies of field variation of the pinning force for
the flux motion across the twin boundaries in YBaCuO single crystals. It is
found that the depinning current J_c decreases with an increase in the magnetic
field due to reduction of the portion of vortex lines trapped by the TB's.
However, at transport currents J << J_c, the vortex velocity weakly decreases
with the increased field, i.e. it is determined mainly by the release of the
vortex lines from the TB's.
