In this paper, an experimental investigation on the heat transfer of saturated water falling film on a single horizontal plain tube is presented. The water film falling on the outside of the tube has been heated by the condensing steam flowing in the tube, and the heat transfer coefficient between the water film and the steam has been measured. Experiments were performed at saturation temperatures of liquid film and steam as 58 degrees C and 61 degrees C, and 61 degrees C and 65 degrees C, a tube pitch of 57.16 mm, heat fluxes from 10 to 50 kW m-2, and film flow rate per unit of length of the tube up to 0.12 kg m-1 s-1. Brass plain tubes with external diameters of 25.4 mm and lengths of 950 mm were used in the experiments. The experimental results show that the heat transfer coefficient increases with the increasing film flow rate and heat flux, and the quality of vapor has an obvious influence on the heat transfer performance of falling film evaporation. The coupling of condensation and evaporation heat transfer inside and outside the tube is investigated qualitatively in this paper.