  We study the charmless two-body decays of $B\to {\bf B}_n \overline{\bf
B}_n^\prime$ with $B = ( B^+ , B^0 , B^0_s)$ and ${\bf B}_n^{(\prime)}$ the
low-lying octet baryons. The factorizable amplitudes are calculated by the
modified bag model, while the nonfactorizable ones are extracted from the
experimental data with the $SU(3)_F$ flavor symmetry. We are able to explain
all current experimental measurements and provide predictions on the decay
branching ratios in $B\to {\bf B}_n \overline{\bf B}_n^\prime$. Particularly,
we find that ${\cal B}(B_s^0 \to \Xi^0 \overline{\Xi}^0) = (19.3\pm 2.7 )\times
10^{-7}$ and ${\cal B}(B_s^0 \to \Xi^- \overline{\Xi}^-) = (19.4\pm 2.7 )\times
10^{-7}$, which are sizable and able to be observed in the ongoing experiments
at LHCb and BELLE-II. Furthermore, the decay branching ratios of $B_s^0 \to
(p\overline{p}, n\overline{n})$ and $B^0 \to (\Xi^0 \overline{\Xi}^0
,\Sigma^+\overline{\Sigma}^+)$ are expected to be $O(10^{-10})$, which are
suppressed due to the angular momentum conservation and chiral symmetry.
