  The Gaussian width of Fermi momentum, $p_{_F}$, is the most important
parameter of the ACCMM model, and its value is essential in the determination
of $|V_{ub}/V_{cb}|$ because the experimental analysis is allowed only at the
end-point region of inclusive semileptonic $B$-decay spectrum. We extract the
value of $|V_{ub}/V_{cb}|$ as a function of $p_{_F}$. We also calculate the
parameter $p_{_F}$ in the relativistic quark model using the variational
method, and obtain $p_{_F} = 0.54$ GeV which is much larger than the commonly
used value, $\sim 0.3$ GeV, in experimental analyses. When we use $p_{_F} =
0.5$ GeV instead of 0.3 GeV, the value of $|V_{ub}/V_{cb}|$ from ACCMM model is
increased by a factor 1.81, and can give a good agreement with Isgur {\it et
al.} model.
