Abstract: Based on the visualized data obtained by the scale-down test facility under external reactor vessel cooling, the image automatic resolution and interface capture program IMGPROCS1 was developed to analyze the observation of downward facing pool boiling phenomena under the hemispherical structure using the commercial software Matlab. Through this image processing program, two phase flow characteristics and morphology along the overheated surface such as interface evolution, vapor film thickness and boiling cycle were quantitatively acquired under different working conditions. The results showed that under nucleate boiling condition, the vapor thickness increased gradually with the increasing of heat flux. The boiling cycle was maintained constant.