In analysing the heat transfer in vertical falling films it has been customary to apply the boundary layer theory in the single-phase turbulent flow. In this paper the limit of the applicability of this method is examined by reviewing papers on this method, and it is estimated that the Reynolds number must not be smaller than 800. Experiments on heat transfer were carried out with liquids of Prandtl numbers between 300 and 5500. By analysing the experimental data it is found that the equivalent eddy thermal diffusivity ε_H/υ is equal to 3 × 10^-5(y⁺)³ for the stable wavy flow, while the order of the first term of the Taylor expansion of the eddy viscosity for the single-phase boundary layer is larger by one ; in other words ε/υ is equal to (const.) x (y⁺)⁴. This difference indicates that, if the Reynolds number is small, the mechanism of the heat transfer in vertical falling liquid films differs largely from that in the flow Which has no free surface