Abstract: Mainly introduces a kind of measuring methods for raindrop size. On the basis of the theory of distance transformation and image segmentation，the raindrop can be mapped to stain pixel，and its diameter is calculated easily. According to the experiments，the relation between the raindrop diameter(d) and the stain diameter (D) is d =0. 356 1× D ^{0.723 3}. This image processing method is highly accurate and efficient because it is not affected by splash burr.

Key words: image processing, raindrop diameter, stain diameter