  We develop perturbative QCD factorization theorem for the semileptonic heavy
baryon decay $\Lambda_b \to \Lambda_c l\bar{\nu}$, whose form factors are
expressed as the convolutions of hard $b$ quark decay amplitudes with universal
$\Lambda_b$ and $\Lambda_c$ baryon wave functions. Large logarithmic
corrections are organized to all orders by the Sudakov resummation, which
renders perturbative expansions more reliable. It is observed that perturbative
QCD is applicable to $\Lambda_b \to \Lambda_c$ decays for velocity transfer
greater than 1.2. Under requirement of heavy quark symmetry, we predict the
branching ratio $B(\Lambda_b \to \Lambda_c l{\bar\nu})\sim 2%$, and determine
the $\Lambda_b$ and $\Lambda_c$ baryon wave functions.
