X-ray microtomography has been shown to be a viable technique for non-destructively and non-intrusively visualizing the three dimensional structure of the entire thickness of paper, paperboard and other porous material. The three dimensional structure of paper plays a critical role in the paper manufacturing process especially via its impact on the transport properties for fluids. Dewatering of the wet web, pressing and drying will benefit from knowledge of the relationships between the web structure and its transport coefficients. A method has been developed to determine the traditional pore structure descriptors such as porosity, fiber-void interface area and pore size distribution for the three dimensional structure. Paper and paper board are complex three dimensional bicontinuum of interconnected pores and cellulose fibers. Transport properties such as vapor diffusivity, liquid permeability and thermal conductivity etc. in porous media are inherently related to the resistance offered by the three-dimensional structure. Brownian motion random walk simulation in actual 3D structures of paper and board obtained by X-ray micro tomography are reported to evaluate transport properties of paper and board.