  We evaluate external $W$-emission, internal $W$-emission and $W$-exchange
contributions to nonleptonic $B\to D^{(*)}$ decays based on the perturbative
QCD formalism including Sudakov effects, whose ratio is found to be
$1:+0.2:0.03i$ at the amplitude level. We observe that the internal
$W$-emission contribution is additive to the external $W$-emission
contribution, and that the $W$-exchange contribution is negligible and mainly
imaginary, its real part being at least one order of magnitude smaller than the
imaginary part. Our predictions are consistent with the CLEO data and with
those obtained by the Bauer-Stech-Wirbel method.
