A large number of small channels, or rills is formed on a bare slope about the same time as surface flow is generated, and the sediment yield rapidly increases as the pattern of rill grows. In this paper we describe an experimental study of the process of growing rill which are formed on composite granite soil, or ”MASA-DO”, by supplying artificial rainfall. We photographed the changes in plane and cross sectional patterns of rills over time and also measured sediment yield. The complexity of rill patterns was expressed numerically using the concept of fractal dimension. As a result of this analysis, the change in fractal dimension of rill patterns over time was closely related to the change in sediment yield and water discharge from the slope.