  We calculate the branching ratios of pure annihilation type decays
$B^0\rightarrow D_{s}^{-}K_{2}^{*+}$ and $B_s\rightarrow \bar{D}a_{2}$ using
the perturbative QCD approach based on $k_T$ factorization. The branching
ratios are predicted to be $(60.6_{-16.5\,-10.4\,-2.1}^{+17.3\,+4.3\,+3.2})
\times 10^{-6}$ for $B^0\rightarrow D_{s}^{-}K_{2}^{*+}$,
$(1.1_{-0.4\,-0.2\,-0.1}^{+0.4\,+0.1\,+0.1})\times 10^{-6}$ for $B_s\rightarrow
\bar{D}a_{2}^{0}$ and $(2.3_{-0.8\,-0.4\,-0.1}^{+0.8\,+0.2\,+0.1})\times
10^{-6}$ for $B_{s}\rightarrow D^{-}a_{2}^{+}$.
  They are large enough to be measured in the ongoing experiment. Due to the
shortage of contributions from penguin operators, there are no direct CP
asymmetries for these decays in the standard model. We also derive simple
relations among these decay channels to reduce theoretical uncertainties for
the experiments to test the accuracy of theory and search of new physics
signal.
