A theoretical model of the effective porosity of coal ash and corresponding experimental data is presented. A new characterization and measurement method to determine the gas diffusion property through the ash layer of a coal particle during the combustion process is proposed. This model encompasses the overall combustion process, which includes transport of oxygen to the outer surface of the coal particle, diffusion of oxygen through the porous ash layer, and heterogeneous combustion reactions on the coal surface. Experimental results show that not all the inner pores of the ash layer are effective for oxygen diffusion, and that the effective diffusion porosity decreases exponentially with the ash-layer thickness. The ash diffusion characteristics of high-ash coal particles play a significant role in the whole combustion process; that is, the role of the ash diffusion of high-ash coal particles cannot be negligible. The proposed measurement method and experimental data are useful to the design, operation, and modeling of low-grade coal-fired fluidized-bed combustion boilers, and to modeling of single-particle coal combustion.