  It is suggested that the measurements of hadronic invariant mass ($m_X$)
distributons in the inclusive $B \rightarrow X_{c(u)} l \nu$ decays can be
useful in extracting the CKM matrix element $|V_{ub}|$. We investigated
hadronic invariant mass distributions within the various theoretical models of
HQET, FAC and chiral lagrangian as well as ACCMM model. It is also emphasized
that the $m_X$ distribution even at the region $m_{X} > m_{D}$ in the inclusive
$b\rightarrow u$ are effetive in selecting the events, experimentally viable at
the future asymmetric $B$ factories, with better theoretical understandings.
