Fragmentation of highly porous particles burning in regime I under two heating modes, uniform and nonuniform, is considered. Oxidation with nonuniform heating is analogous to regime II, because it creates nonuniform reactivity. A uniformly heated particle did not fragment, even for large extents of burning. The nonuniformly heated ones showed systematic local fragmentation. Analysis of a particle's behavior under the two heating modes allowed one to substantiate that percolation fragmentation does not occur by a change in total porosity, but is due to changes in the larger pores of the char (meso- and macro-). Calculations were compared with experimental data from the literature and the following conclusions were drawn: (1) Fragmentation is determined by the porosity of the larger pores with diameters higher than the mean dimension of a microcrystal within the microskeleton. (2) Shrinkage is determined by the micromedium, or small pores. (3) For uniformly heated particles the porosity of the large pores barely changes during burning, which accounts for nonfragmentation. (4) The reason for local fragmentation under nonuniform oxidation is the increase in the local macroporosity, triggered by nonuniform shrinkage of the micromedium.