The porous materials are usually the sources or sinks of indoor volatile organic compounds (VOC). In order to predict the VOC emission rate and choose best adsorbents, reasonable estimation of the diffusion coefficients of porous materials is required. Porous materials were usually depicted as a random network of interconnected pores of unspecified sizes and shapes. It may generate deviation to assume the network as simple topological structures. This paper investigated the diffusion coefficients of porous materials with the fractal theory and derived their diffusion coefficients considering the pore size distribution of the materials. The calculated result was validated by published experimental data.