  We present improved measurements of the branching fractions for the decays B0
--> Ds+pi- and B0-bar --> Ds+K- using a data sample of 657x10^6 BB-bar events
collected at the Y(4S) resonance with the Belle detector at the KEKB
asymmetric-energy e+e- collider. The results are BF(B0 --> Ds+pi-) = (1.99 +/-
0.26 +/- 0.18)x10^-5 and BF(B0-bar --> Ds+K-) = (1.91 +/- 0.24 +/- 0.17)x10^-5,
where the uncertainties are statistical and systematic, respectively. Based on
these results, we determine the ratio between amplitudes of the doubly Cabibbo
suppressed decay B0 --> D+pi- and the Cabibbo favored decay B0 --> D-pi+, R_Dpi
= [1.71 +/- 0.11(stat) +/- 0.09(syst) +/- 0.02(theo)]%, where the last term
denotes the theory error.
