Abstract The evolution of pore structures during coal devolatilization was studied using the mercury porosimetry to classify pore structures as micro pores or macro pores based on fractal theory. Pyrolysis caused few changes in the total specific surface areas and micro pore surface areas of most samples. However, the specific macro pore surface areas and the ratio of the specific macro pore surface areas to the total specific surface areas were significantly increased, mostly a result of the shapes of some micro pores becoming less complex after pyrolysis. After devolatilization, the fractal dimensions of the macro pores, which reflect the complexity of the geometrical shapes and hence, the resistances to gas diffusion increased. The fractal geometric factor β, which combines the effects of the ratio of the macro pore surface area to the total surface areas and the fractal dimension of the macro pores after char combustion presented in the previous study is also increased. The volatile content in the parent coal affects the changes of the pore structures.