Abstract: Some fractal characters of storm sewer system are demonstrated, i. e. the part of such a system composes the information of the whole system on a certain degree, and there is statistically self-similarity. Therefore, fractal theory is introduced to the research of storm sewer networks to analyse their fractal character. Choosing an algorithm——box-counting algorithm and applying it to the study of some storm sewer system in some Beijing suburban districts or towns, the authors have drawn out their fractal dimensions. Based on fractal study in rivers and in roads, the correlation and restriction between fractal dimensions and runoff modulus are discussed. They penetrate the concealed information beneath fractal dimension and conclude that the optimal layout maybe exist.