  The two important issues concerning the theory of inclusive decays are
studied: (1) evaluation of expectation values of four-quark operators in order
to systematically include the spectator effects and (2) estimation of duality
breaking corrections in heavy quark expansion. Our study leads to the
theoretical prediction of the ratio of lifetimes of $\Lambda_b$ and $B$ close
to the experimental one and the duality breaking corrections of about 10% of
the leading decay rate.
