Grain size distribution of bed material is an important characteristic for studying evolution of natural river channel by means of experimental ways and numerical modeling of flow and sediment process.In this study,the fractal characteristic of sediment particle has been defined by means of fractal theory based on ana- lyzing the property of grain size distribution of bed material in the river channel.Furthennore,the fractal prop- erty of sediment particle has been applied to judge the process of armoring and stabilizing in the armoring ex- periment of bed material.In addition,the relationship between the riverbed roughness and its fractal dimension is also obtained according to the experimental data.Finally,the relationships between bed material roughness, fractal dimension,and representative diameter(d_(50),d_(90))are discussed respectively,the roughness comparison shows that the proposed roughness formula based on fractal theory has higher precision. Grain size distribution of bed material is an important characteristic for studying evolution of natural river channel by means of experimental ways and numerical modeling of flow and sediment process. In this study, the fractal characteristic of sediment particle has been defined by means of fractal theory based on ana- lyzing the property of grain size distribution of bed material in the river channel. Fachennore, the fractal prop-erty of sediment particle has been applied to judge the process of armoring and stabilizing in the armoring ex- periment of bed material. addition, the relationship between the riverbed roughness and its fractal dimension is also obtained according to the experimental data .Finally, the relationships between bed material roughness, fractal dimension, and representative diameter (d_ (50), d_ (90)) are discussed respectively the roughness comparison shows that the proposed roughness formula based on fractal theory has higher precision.