As key factor to the heat transfer efficiency of the gravity film exchanger, the wave characteristics of gravity films were studied in the experimental setup. The water from the high level storage tank flowed down the vertical circular tube to form the gravity films. The images of the gravity films were captured with the aids of the plane light source and the high speed image technology, and recorded in computers in real time. With the image processing software, the wave profiles varied with time and the height of the tube were both shaped from the images of film thickness fluctuations. The experimental results showed the significant effect of the film Reynolds numbers on the wave characteristics in various positions of the vertical tube. The probability density distributions of the film thickness were also analyzed for different Reynolds numbers.