  We present a systematical study of two-body and three-body charmless baryonic
B decays. Branching ratios for two-body modes are in general very small,
typically less than $10^{-6}$, except that $\B(B^-\to p \bar\Delta^{--})\sim
1\times 10^{-6}$. In general, $\bar B\to N\bar\Delta>\bar B\to N\bar N$ due to
the large coupling constant for $\Sigma_b\to B\Delta$. For three-body modes we
focus on octet baryon final states. The leading three-dominated modes are $\bar
B^0\to p\bar n\pi^-(\rho^-), n\bar p\pi^+(\rho^+)$ with a branching ratio of
order $3\times 10^{-6}$ for $\bar B^0\to p\bar n\pi^-$ and $8\times 10^{-6}$
for $\bar B^0\to p\bar n\rho^-$. The penguin-dominated decays with strangeness
in the meson, e.g., $B^-\to p\bar p K^{-(*)}$ and $\bar B^0\to p\bar n
K^{-(*)}, n\bar n \bar K^{0(*)}$, have appreciable rates and the $N\bar N$ mass
spectrum peaks at low mass. The penguin-dominated modes containing a strange
baryon, e.g., $\bar B^0\to \Sigma^0\bar p\pi^+, \Sigma^-\bar n\pi^+$, have
branching ratios of order $(1\sim 4)\times 10^{-6}$. In contrast, the decay
rate of $\bar B^0\to\Lambda\bar p\pi^+$ is smaller. We explain why some of
charmless three-body final states in which baryon-antibaryon pair production is
accompanied by a meson have a larger rate than their two-body counterparts:
either the pole diagrams for the former have an anti-triplet bottom baryon
intermediate state, which has a large coupling to the $B$ meson and the
nucleon, or they are dominated by the factorizable external $W$-emission
process.
