The concepts of fractal dimension and spectrum analysis have been used for the numerization of the very complex patterns of the natural world. In this paper, a development model of rill patterns was made with the aid of a random pattern model, which is used in the field of fractal geometry. The rill patterns, that develop on hillslopes, were simulated by a personal computer based on this model. Though the development of the actual rill depends considerably on many factors, such as soil, hydraulics, rainfall, and the topographic conditions. The fractal dimensions and the entropy which is characteristics of a power spectrum, are all within range of the values of an actual rill.