This paper investigates the flow of a thin liquid film on the lower surface of a horizontal rectangular duct subjected to forced vibration. The duct had a cross-section of 40 x 30 mm(2) and a length of 1000 mm. The instantaneous film thickness recorded by electrode-type film thickness gauges for various conditions of steam flow and quality was used for the analysis of the mean characteristics of the thin liquid film. Statistical analysis was carried out to understand the effect of vibration on the wave structure of the film. The results show that, in the low-frequency zone (f < 100 Hz), which corresponds to the maximum amplitude of vibration, the thickness of the liquid film decreases and the film surface is disturbed. The presence of a surface active agent in the steam helps to stabilize the wavyness of the surface.