Pore-network is a useful tool in the study of porous media. It makes it possible to exam the structure of a porous media and to carry out simulations in a macroscopic way. For instance, with a pore-network, the calculation of permeability can be done by implementing Darcy’s law, a formula in a proportional form. In this paper, the inscribed sphere algorithm is introduced to extract the pore-network from micro-CT images. Also methods for determining shape factor and conductance are introduced. Several samples with different porosity are used for test. Absolute permeability is calculated based on two different ways of evaluating conductance, and the results are compared and analyzed.