  Heavy quark expansion can nicely explain the lifetime of $\Lambda_{b}$.
However, there still exsit sizable uncertainties from the four-quark operator
matrix elements in the power of $1/m_{b}^{3}$. In this work, the leading order
results of the four-quark operator matrix elements
$\langle\Lambda_{b}|(\bar{b}q)_{V-A}(\bar{q}b)_{V-A}|\Lambda_{b}\rangle$ and
$\langle\Lambda_{b}|(\bar{b}b)_{V-A}(\bar{q}q)_{V-A}|\Lambda_{b}\rangle$ are
obtained using full QCD sum rules. Contributions from dimension-0,3,5 are
considered. It turns out that the contributions from dimension-3,5 are
proportional to the mass of $u/d$ quark. Stable Borel region can be found, and
for this reason, the uncertainties caused by the QCD sum rule parameters are
small. The leading logarithmic corrections are also considered, which turn out
to be a little destructive. Our results are close to the lower bound of the
existing theoretical predictions.
