  In this work, we evaluate the contributions of non-spectator effects to the
lifetimes of $\Lambda_b$ and B-mesons. Based on the well-established models and
within a reasonable range of the concerned parameters, the contributions can
reduce the lifetime of $\Lambda_b$ by $7 \sim 8%$ compared to that of B-mesons
which are not significantly affected. This might partly explain the measured
ratio $\tau(\Lambda_{b})/\tau(B^{0})=0.79 $ \cite{Data}, which has been a
long-standing discrepancy between theory and experimental data.
