  We in terms of optical theorem estimate the lifetime of $B_c-$meson with the
parameters which are deturmined by fitting the data for the lifetimes and
inclusive semilepton-decays of various $B$ and $D$ mesons. In the estimate, we
find that the bound-state effects are important, and take them into account
carefully in the framework which attributes the effects to the effective masses
of the decay heavy quarks in the inclusive processes. We also find that to
$B_c$ lifetime the penguin contribution is enhanced due to possible
interference between the penguin and the `tree part' $c_1O_1+c_2O_2$.
