The surface properties and pore structure of minerals/rocks are key parameters controlling their geochemical dynamic behaviors. Characterizing these properties on the nanometer scale is greatly significant for geochemical researches and development of new materials. Fractal dimension is an integrated parameter to describe quantitatively the roughness of the surface and the pore-structure in the geological materials. A method for fractal study based on gas adsorption isotherms is introduced in this paper. The suitability and advantages are discussed in the view of the theoretical argument. Based on the review of application of this method in organic petrology, pedology and material sciences, the application prospect of this method in earth science is schemed.