  The QCD-based operator-product-expansion technique is systematically applied
to the study of charmed meson lifetimes. We stress that it is crucial to take
into account the momentum of the spectator light quark of charmed mesons,
otherwise the destructive Pauli-interference effect in $D^+$ decays will lead
to a negative decay width for the $D^+$. We have applied the QCD sum rule
approach to estimate the hadronic matrix elements of color-singlet and
color-octet 4-quark operators relevant to nonleptonic inclusive $D$ decays. The
lifetime of $D_s^+$ is found to be longer than that of $D^0$ because the latter
receives a constructive $W$-exchange contribution, whereas the hadronic
annihilation and leptonic contributions to the former are compensated by the
Pauli interference. We obtain the lifetime ratio $\tau(D_s^+)/\tau(D^0)$
$\approx 1.08\pm 0.04$, which is larger than some earlier theoretical
estimates, but still smaller than the recent measurements by CLEO and E791.
