Abstract: The permeability of coalbed benefits the right evaluation of gas drainage capability, which could be used to select the suitable technique to increase the permeability and drainage effect. The permeability of coalbed is evaluated referring to the theory and experience of gas pressure build-up in the well testing. The gas flow model appropriate to the underground coal mine has been set up. The starting point of radial flow in the pressure build-up curve could be designated rationally using the double and half logarithmic pressure build-up curves with the analytic method of half logarithmic straight line. Therefore the slope of the interim radial flow section and the permeability of the coalbed could be determined accurately. After the verification and comparison with the result of in-situ examples by the radial flow method, the relative error of the five examples was between 13% and 36.6% and the average was lower than 24.62%. So the permeability of coalbed by the borehole gas pressure build-up curve is practical.