  We study semi-inclusive charmless decays $B \to \pi X$, where $X$ does not
contain a charm (anti)quark. The mode $\bar B^0 \to \pi^- X$ turns out to be be
particularly useful for determination of the CKM matrix element $|V_{ub}|$. We
present the branching ratio (BR) of $\bar B^0 \to \pi^- X$ as a function of
$|V_{ub}|$, with an estimation of possible uncertainty. The BR is expected to
be an order of $10^{-4}$.
