  The dilational viscosity $\epsilon'$ of Langmuir monolayer is considered in a
theoretical model taking into account an orientational effect of the dilational
wave on surface molecules. This orientational order is described by the surface
order parameter Q; the orientational part of the free surface energy is given
by Landau's expansion in powers of Q. The magnitude of surface viscosity,
driven by the surface tension derivative $\partial\gamma/\partial Q$, is in
good accord with the experimentally observed $\epsilon'$. The sign of
$\epsilon'$ is positive that indicates that increased in-plane ordering
increases the surface tension.
