  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. For $eb \to t \bar{\nu}$ subprocess we show that
the top quark spin is completely polarized when the spin basis is chosen in the
direction of the incoming positron beam in the rest frame of top quark. 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.
