  Uncertainty in the PQCD calculation of B decays is investigated in $B\to
\pi$, $B\to D$ transition form factors and $B\to D\pi$ decay amplitudes. $B$
meson distributionamplitude dependence is studied by taking three kinds of
distribution amplitude so far suggested. It is found that almost same $q^2$
dependence of the form factors can be obtained irrespective of the types of the
$B$ meson distribution amplitude by suitably choosing one parameter. $B\to
D\pi$ process shows the difference due to the distribution amplitude. The
effect of the sub-leading component of the $B$ mesondistribution amplitude is
also studied in the three processes. The numerical results of calculations with
the sub-leading component can be well approximated by the leading order
calculation with a suitable choice of the distribution amplitude parameters.
