  We study semi-inclusive charmless decays $B \to \pi X$ in detail, such as
$\bar B^0 \to \pi^{\pm (0)} X$, $B^0 \to \pi^{\pm (0)} X$, $B^{\pm} \to
\pi^{\pm (0)} X$, where $X$ does not contain a charm (anti)quark. We find that
the process $\bar B^0 \to \pi^- X$ ($B^0 \to \pi^+ X$) can be particularly
useful for determination of the CKM matrix element $|V_{ub}|$. We calculate and
present the branching ratio (BR) of $\bar B^0 \to \pi^- X$ as a function of
$|V_{ub}|$, with an estimate of possible uncertainties. It is expected that the
BR is an order of $10^{-4}$. Our estimation indicates that one can
phenomenologically determine $|V_{ub}|$ with reasonable accuracy by measuring
the BR of $\bar B^0 \to \pi^- X$ ($B^0 \to \pi^+ X$).
