  \Lambda_b matrix element of four-quark operator relevant to its lifetime is
reanalyzed by QCD sum rule. The new ingredients introduced are that (1) more
vacuum condensates are considered; (2) different quark-hadron duality is
adopted; and (3) the possible deviation from the vacuum saturation assumption
for the four quark condensates is considered. With \kappa_1=4 the related
hadronic parameter r is calculated to be 3.6 \pm 0.9, and the lifetime ratio
\tau(\Lambda_b)/\tau(B^0) = 0.83 \pm 0.04, which is consistent with the
experimental data.
