CCAST, World Lab, Beijing 100080, Peoples R China
; CAS, Div Theory, Inst High Energy Phys, Beijing 100039, Peoples R China
; Nagoya Univ, Dept Phys, Nagoya, Aichi 4648602, Japan
The rare decays B-0 --> D-s(-) K+ and B+ --> D-s(+)(K) over bar (0) can occur only via annihilation type diagrams in the standard model. We calculate these decays in the perturbative QCD approach. We found that the calculated branching ratio of B-0 --> Ds-K+ agreed with the data which had been observed in the KEK and SLAC B factories. The decay B+ --> D-s(+)(K) over bar (0) has a very small branching ratio at O(10(-8)), due to the suppression from the CKM matrix elements \V-ub* V-cd\.