We present improved measurements of the branching fractions for the decays B-0 -> D-s(+) pi(-) and (B) over bar (0) -> D-s(+) K- using a data sample of 657 X 10(6) B (B) over bar events collected at the Y(4S) resonance with the Belle detector at the KEKB asymmetric- energy e(+)e(-) collider. The results are B(B-0 -> D-s(+) pi(-)) = (1.99 +/- 0.26 +/- 0.18) X 10(-5) and B((B) over bar (0) -> D-s(+) K-) = (1.91 +/- 0.24 +/- 0.17) X 10(-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 B-0 -> D+pi(-) and the Cabibbo favored decay B-0 -> D-pi(+), R-D pi = [1.71 +/- 0.11(stat) +/- 0.09(syst) +/- 0.02(theo)]%, where the last term denotes the theory error.