Porosity variation of porous medium consisting of screen mesh layers is determined experimentally layer by layer. Volume element which is that the porosity calculated with respect to the increasing volume elements becomes constant at any point within the medium, is round out by the help of experimental data. The volume element that gives the same result approximately at each point of the medium except wall region, is considered as REV of that medium. By determining the smallest volume element which is that the porosity is not depended upon volume at every point, the selection criterion of REV is applied partially. On the other hand, it is assumed that the porosity varies exponentially in the wall region. The appropriate values of the dimensionless coefficients of that exponential function are found out by the experimental data and REV. (C) 1997 Elsevier Science Ltd.