Using a compartment with adjustable oxygen concentration and internal pressure, the shrinkage and cracking of isotropic char layer during pyrolysis were studied by pyrolyzing the medium density fibreboard (MDF) in inert atmosphere with different ambient pressures. The experimental results have shown that the ambient pressure has insignificant effect on shrinkage although some trends have been identified. Therefore, the reduction of tensile strength of material dominates the char cracking under low pressures, leading to noticeableincrease in the number of char fissures. The char shrinkage could expose the raw material beneath the char layer and enhance the radiative heat transfer at the sample surface (by fissure width instead of number), which is modeled using a modified thermal conductivity as the typical simplification. It is found that the mass loss rate at the early pyrolysis stage increases up to 20% with increasing width of char fissures. However, the external radiation can only affect a limited depth in the near surface zone which is found less than 3 mm in the current experiments. The thermal conductivity under near regular pressure could be over 2 times higher than its value under low pressure (30 kPa) and with which the differences in the first peaks of mass loss rate under different pressures can be well predicted.