  We discuss the degree of spin polarization of single top quarks produced via
$Wg$ fusion process in $ep$ collision at TESLA+HERAp and CLIC+LHC energies
$\sqrt{s}=1.6$ and 5.3 TeV. A description on how to combine the cross sections
of $e^{+}b\to t\bar{\nu}$ and $e^{+}g\to t\bar{b}\bar{\nu}$ processes is given.
$e^{+}$-beam direction is taken to be the favorite top quark spin decomposition
axis in its rest frame and it is found to be comparable with the ones in $pp$
collision. It is argued that theoretical simplicity and experimental clearness
are the advantage of $ep$ collision.
