A mathematical model based on unreacted-core shrinking model is developed to describe the pyrolysis phenomena of the cellulosic materials in the temperature range of 700–1470°K. The relative importance of each model parameter is studied by applying the sensitivity analysis. The heat transfer control region and kinetic reaction control region are obtained from the developed model. The shift of controlling mechanism is dependent on the particle size and the wall temperature. The effect of heat of reaction on the temperature of the particle is also discussed.