  A simple analytical microscopic expression for the surface tension of liquids
$\gamma$ is obtained which is in a good agreement with available data of
numerical experiments. We apply the integral transformation that maps the fluid
Hamiltonian onto the field-theoretical Hamiltonian and show that the order
parameter of the effective Hamiltonian corresponds to the one-body potential in
the fluid. Revealing the physical meaning of the order parameter allows
calculation of $\gamma$.
