We performed Monte Carlo simulations in the canonical ensemble on the liquid-vapor interface of a square well fluid with interaction range of lambda=1.5sigma. The system contains a liquid slab surrounded by vapor. The surface tension is calculated during simulations by using an original procedure that allows the calculation of the pressure tensor components. The surface tension decreases monotonically with temperature. Coexisting densities and pressure along the liquid-vapor coexistence line have also been obtained and good agreement is found with results calculated from bulk simulations. (C) 2003 American Institute of Physics.