  The $W$-exchange process $B_{s}$ ${\to}$ ${\eta}_{c}(J/{\psi})D$ is studied
with the perturbative QCD approach. Three kinds of wave functions for $B_{s}$
meson and two forms of wave functions for charmonium are considered. It is
estimated that branching ratios for $B_{s}$ ${\to}$ ${\eta}_{c}\bar{D}$,
${\eta}_{c}D$, $J/{\psi}\bar{D}$, $J/{\psi}D$ decays are the order of
$10^{-7}$, $10^{-8}$, $10^{-8}$, $10^{-9}$, respectively, where the largest
uncertainty is from wave functions. There is a possibility for measuring these
decay in the near future.
