The motion of the wave arising on the liquid-gas interface (so-called Taylor instability) during the film flow down a horizontal cylinder has been analysed. The results of calculation of the wavelength from the formula obtained were compared with those calculated from the Lienhard-Wong＇s expression as well as with the results of experiments. A good agreement between the calculated and experimental values has been achieved. Some results of the calculation of the wavelength for the film flow of water, ammonia and freon R22 on cylindrical surfaces of small curvature radii have been presented.