As a new technology, Atomil Force Microscopy (AFM) is being used in the research of microscopic structure on coal surface in recent years. By this technology, we can observe the nanoscale pore and crack shape of coal surface, and measure some structural parameters. Different metamorphic grades produce different feature of surface microscopic structure of coal. This paper analyzes the surface microscopic structure of different metamorphic grade coal by AFM. The results show that the coal surface microstructure has a trend from rough to smooth with the increasing of metamorphic grade. The low rank coals contain large or medium pores and the high rank coals contain micro pores. The values of surface morphology characteristic parameters (S_q and S_a) nonlinearly decrease with the increasing coal rank. That is, the coal surface becomes smoother during coalification.