  In order to study the end-point spectrum of inclusive semileptonic $b
\rightarrow u$ decays, we investigate the parton model motivated by deep
inelastic scattering process. The reliability of the model is carefully
examined by comparing the decay rates and the lepton spectra for $b \rightarrow
c$ decay with those of the heavy quark effective theory. We also compute the
decay rates of the inclusive semileptonic decay into tau lepton in terms of the
parton model, which give another test of the model independently. We conclude
that one can make the parton model consistent with the heavy quark effective
theory, and it would be phenomenologically useful in extracting $|V_{ub}|$.
