  We develop the perturbative QCD formalism including Sudakov effects for
semi-leptonic $B$ meson decays. We evaluate the differential decay rate of
$B\to \pi l \nu$, and find that the perturbative calculation is reliable for
the energy fraction of the pion above 0.3. Combining predictions from the soft
pion theorems, we extract the value of the matrix element $|V_{ub}|$ which is
roughly $2.7\times 10^{-3}$.
