Abstract: According to the physical characteristics of gas-liquid interfaces, the film thickness on the perimeter of a horizontal tube was experimentally studied by a conductance probe method to understand the flow behavior of a falling film. Measurements were conducted under the following conditions: outside diameter D=20—32 mm, intertube spacing S=10—40 mm, and film Reynolds number Re=150—800. Falling film distributions over a range of circumferential angles from 10° to 170° were obtained and their effects on heat transfer were examined and analyzed. The results indicate that the values predicted by Nusselt's theory are larger than the measured values on the lower perimeter of the tube. The minimal values of the film thickness tend to locate at different circumferential angles in the range of 90° to 115° under different operating conditions. The film thickness decreases with the intertube spacing increasing and increases with Re. Moreover, the changes in tube diameter have little influence on the film distribution. The rough outer wall of the tubes can make the film thinner. In addition, the film free-surface is disrupted in the region of scale formation, which facilitates the formation of dry patches on the heat transfer surface.