Abstract: A new characterization method for nanofibers membrane was put forward based on the digital image processing and analysis technology. Firstly, segmentation threshold of SEM images were obtained by Photoshop, and then the areas were counted by Matlab. And finally the value of porosity was acquired according to calculating. Test results showed that the porosity of nanofiber membrane decreases with fiber diameter increased. Meanwhile,the results demonstrated that the errors of samples were less than 6% compared with density method. Furthermore, in order to research the influence of the fiber crisscross on porosity, membranes with different thickness were prepared and tested in the same conditions, and it was found that the errors between samples were less than 4% ,therefore,fiber crisscross had little influence on the measurement results might be considered.