A model of solid particle, based on fractal geometry of surfaces, was developed to describe the distribution of radioactivity in the natural environment. Two-fold distribution of natural radionuclides was assumed: internal and in the surface layer of the particle. The fractal-particle model was fitted to two sets of literature data on radioactivity distribution in coal fly ash particles and three sets of data for soil particles. In all cases studied, it was possible to determine the fractal dimension of the particle surface as a measure of its roughness. The values ranged from 2.40 to 2.79 for coal fly ash and from 2.50 to 2.73 for soil. Internal radioactivity concentrations were also determined. In some cases, it was possible to determine the surface concentrations of radioactive elements as well as an average thickness of the surface layer. Most of the radionuclides were found to be predominant on the surfaces of the particles, whereas in some cases, 40K, 226, 228Ra showed a depletion on the surfaces. The criteria for experimental data best suitable for the model are described, and new experiments to study roughness of environmental particles using radioactive tracers are proposed.