We calculate the branching ratios of pure annihilation type decays B-0 -> Ds-K2*(+) and B-s ->(D) over bara(2) using the perturbative QCD approach based on kT factorization. The branching ratios are predicted to be (60.6(-16.5-10.4-2.1)(+17.3+4.3+3.2)) x 10(-6) for B-0 -> Ds-K2*(+), (1.1(-0.4-0.2-0.1)(+0.4+0.1+0.1)) x10(-6) for Bs ->(D) over bar (00)(a2) and (2.3(-0.8-0.4-0.1)(+0.8+0.2+0.1)) x10(-6) for B-s -> 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 to search for a signal of new physics.