Abstract: Coal permeability is a critical parameter for characterizing coalbed methane reservoirs and predicting their production; however, the role of the directional permeability in coalbed methane prediction has been neglected in previous studies. This study is focused on directional permeability, its existance is confirmed experimentally and a method is presented to determine the maximum and minimum principal permeability. In addition, the results showed that directional permeability measured on different coal samples varies significantly and is dependent on the effective stress. After determining the maximum and minimum principal permeability, the only functional relationship between the directional permeability and the effective stress is finally determined, which is helpful to accurately evaluate coalbed methane reservoirs.