Important parameters in the modeling of the pore structure of graphite are average diameter and number density of distributed capillary tubes in the bulk. However, in order to describe theoretically pore structure elements such as pore volume and gas permeability, average square diameter and average cubic diameter are newly defined. Then formulations for surface roughness factor, integral pore volume and gas permeability are made. As pore parameters of graphite pore size and integral pore volume are reported by manufacturers. Values of surface roughness factors and gas permeabilities are calculated by putting these parameter values into the formulas and compared with available experimental data for the two elements. As a conclusion pore size takes generally lager value by factor of about 3 than average diameter defined in this modeling.