Abstract: Debris flow can be seen as a two-phase rheological fluid with soil-fluid compositions,and the debris-flow deposits consist of mud,silt,sandy gravel,boulder,etc.,with grain sizes ranging from several microns to more than tens of meters.Consequently,solid grains in the debris-flow deposits display a marked self-similarity in geometrical shape and scale-invariance in size according to fractal theory.In this paper,various debris-flow deposits located in the Xiaojiang River basin were collected as studied samples,and the features of grain-size distribution of these deposits are examined through illustration method.Then fractal dimensions of solid grains in these deposits are calculated respectively with the dimension values between 2.5 and 3,showing obvious fractal features of grain-size distribution.Furthermore,the results of grain-size dimensions are consistent with the results obtained through the former method.So fractal dimension can be seen as such a parameter,which reflects the grain composition and grain-size distribution of the debris-flow deposits.Finally,the relationship between fractal dimension,and viscosity and accumulation ages of debris-flow deposits are discussed as more characteristics of debris flow are determined by grain composition and grain-size distribution.