Abstract: To explore the bio-permeability enhancing effect via methanogen treatment on different rank coals after anaerobic fermentation, micro-CT was used to analyze the change characteristics of pore structure of coal before and after biodegradation. At the same time, the adsorption characteristics of methanogens on the coal surface after anaerobic biodegradation were observed by scanning electron microscopy to analyze the reasons for the effect difference of biological transformation of different rank coals. The results show that the porosity and total throat length of the coal samples biodegraded increase and the connectivity of the pores is enhanced, it is indicated that coal samples were degraded by microorganisms, which promotes the formation, expansion and penetration of new pore in the coal samples, thus realizing the bio-permeability enhancing effect of coal. With the decrease of coal rank, the improvement of pore structure is more obvious. The main reason for the change is that the surface of low rank coal is more suitable for the adsorption of methanogens. The research can provide a theoretical reference for the domestic coalbed biogenic gas production.