The nanofibrous structures have received considerable attention in recent years. Because of the extraordinary features such as high porosity, high specific area and desirable functionality; these structures are used in various applications such as filtration and separation, advanced materials, catalysts, drug delivery and tissue engineering, etc... . Suitability of nanofibrous structure for any particular application is highly dependent on its structural characteristics such as porosity. In a nanofibrous structure, nanofiber morphology, pores formed between the nanofibers and the structure permeability are the most important variables that determine its performance. In this study, the air permeability of electrospun poly acrylonitrile nanoweb was evaluated both by theory and experiment considering the pores formed between nanofibers. To achieve this, poly acrylonitrile nanofibers were electrospun in different process and solution conditions. Collecting electrospun nanofibers on paper frame and aluminum foil provide proper samples for air permeability test and scanning electron microscope respectively. By processing the images obtained from scanning electron microscope, the nanofibrous layer structure specifications including nanofiber diameter and nanofiber diameter distribution, number of pores formed between nanofibers, pores area, and pores size distribution were determined. The effect of polymeric solution and electrospinning process variables on the nanofibrous web, structural specifications and air permeability were assessed for further understanding of the relationship between structure geometry and permeability. In the theoretical section, two geometrical models based on geometric probability and Poisson process in two-dimensional space were presented to evaluate the size of the pores formed between the nanofibers. Prediction of both models for the mean radius of pores formed between the nanofibers were compared with the results of image processing and the results of porosimetery. Finally, separate statistical models were presented to predict the air permeability of nanoweb using the data extracted from image processing and also using the electrospinning process data. Both models can explain well the air permeability of nanoweb in the range of mentioned variables. The importance of first model is it ability to determine the air permeability of nanofibrous web using the data from SEM image processing with no need to do air permeability test which is difficult and complex for tiny nanofibrous web samples. The importance of second model is its ability to produce nanofibrous web with certain air permeability.