The pore structure in char particles changes greatly during combustion, which significantly affects the char combustion characteristics. This article describes a model for the evolution of the char pore structure. A fractal pore model is used to generate the porous char particles to simulate the char combustion for various porosities, theta, specific surface areas, S-g, fractal dimensions, D-f, and particle diameters, d(p). Then, expressions are given to associate these pore structure parameters with the char conversion, X, and the operating conditions, using a modulus, chi, indicating the pore diffusion resistance. As X increases, theta increases, D-f and d(p) decrease, and S-g increases to a maximum first and then decreases. In some cases, S-g only decreases with increasing X when the reaction rates are too high or the conditions are such that the O-2 cannot diffuse into the particle. The results compare well with the pore structure parameters of three Chinese chars combusted in a drop tube furnace at different conversions measured by a mercury porosimeter.