  We develop the perturbative QCD formalism for inclusive semileptonic $B$
meson decays, which includes Sudakov suppression from the resummation of large
radiative corrections near the high end of charged lepton energy. Transverse
degrees of freedom of partons are introduced to facilitate the factorization of
$B$ meson decays. Ambiguities appearing in the quark-level analysis are then
avoided. A universal distribution function, arising from the nonperturbative
Fermi motion of the $b$ quark, is constructed according to the heavy quark
effective field theory based operator product expansion, through which the mean
and the width of the distribution function are related to hadronic matrix
elements of local operators. Charged lepton spectra of the $B\to X_u\ell\nu$
decay are presented. We find 50\% suppression near the end point of the
spectrum. The overall suppression on the total decay rate is 8\% for the free
quark model, and is less than 7\% for the use of smooth distribution functions.
With our predictions, it is then possible to extract the
Cabibbo-Kobayashi-Maskawa matrix element $|V_{ub}|$ from experimental data. We
also discuss possible implications of our analysis when confronted with the
rather small observed semileptonic branching ratio in $B$ meson decays.
