Effects of radial diffusion on the hydrogen separation from a low-flow-rate gas mixture containing hydrogen was examined under atmospheric pressure at 473 K using an annular packed-bed type of palladium membrane reactor. The axial profiles of hydrogen concentration (partial pressure) in the packed bed were experimentally measured and mathematically analyzed using two flow models, ideal and radial diffusion models. The ideal flow model could not explain the experimental results very well. On the other hand, the radial diffusion model could successfully simulate the profiles of hydrogen partial pressure formed along the packed bed of palladium membrane reactor. It became clear that there exists a gradient of hydrogen partial pressure in the radial direction of the packed bed due to its permeation through palladium membrane. In addition, Peclet numbers at low Reynolds numbers were determined and were explainable by the extended Wen and Fan equation.