On the basis of a study of Carboniferous coal measures in the northern foothills of the Dabie orogenic belt and by using the microscopic and ultramicroscopic techniques, the structural coal is subdivided into brittle-deformed coal and ductile-deformed coal. X-ray diffraction analysis indicates that brittle deformation cannot bring about the change of the coal rank, but ductile deformation can and that temperatures are the main factor for raising the coal rank and the directional pressure acts as the "catalysis" in the process of coalification.