  The rare decays $B^+ \to D_s^{+} \phi$, $B^0 \to D_s^{(*)-} K^+$ and $B^+ \to
D_s^{(*)+} \bar{K}^0$ can occur only via annihilation type diagrams in the
standard model. We calculate these decays in perturbative QCD approach. We
found that the calculated branching ratio of $B^0 \to D_s^- K^+$ agree with the
data which had been observed in the KEK and SLAC B factories. The decay $B^+
\to D_s^{(*)+} \bar K^0$ has a very small branching ratio at O ($10^{-8}$), due
to the suppression from CKM matrix elements. The branching ratio of $B^+ \to
D_s^+ \phi$ is of order $10^{-7}$ which may be measured in the near future by
KEK and SLAC B factories. The small branching ratios predicted in the standard
model make these channel sensitive to new physics contributions.
