Experiment is carried out to observe the structural changes of cellulosic materials exposed to thermal radiation. To quantitatively analyze the results of pyrolyzed wood samples, simple geometric models are proposed to include both shrinkage and cracking. The shrinkage factors consider each direction of three-dimensional geometry, as well as the overall volume. The cracking coefficients include the depth and volume of cracks. The structural changes of the solid are characterized during the pyrolysis by varying the external heat flux and the species of wood. Formulas for calculation of crack depths are derived based on the experimental results for softwood and hardwood. The expressions of cracking can be incorporated into pyrolysis models to include the effects of cracks on heat and mass transfer during wood pyrolysis.