In electrospinning, the structure of nanofibers, which is affected by polymer solution parameters and processing conditions, influences the physical characteristics of nanofiber mats. In this study, under optimum conditions of electrospinning, the concentration of polyacrylonitrile (PAN) was changed from 11 to 15 wt %, and its effects on the nanofiber diameter and surface porosity of nanofiber mats were studied. The results showed that increasing the PAN polymer concentration enhanced the nanofiber diameter but reduced the surface porosity of nanofiber mats. Because the diameter and surface porosity are parameters that possess mutual effects, a structural parameter (Q) was introduced, and then its relation to some of the physical characteristics, such as the air permeability and surface roughness, was investigated. To evaluate the surface roughness, atomic force microscopy (AFM) and entropy (ENT) methods were used. The surface roughness of nanofiber mats was measured by AFM in a surface non-contact mode and by ENT with an image analysis technique and co-occurrence matrix. The correlation coefficient of the surface roughness obtained from these two methods was 940%. The results also present a strong dependence between Q and the surface roughness and air permeability of nanofiber mats. (C) 2008 Wiley Periodicals, Inc.