In this paper parameters quantitatively characterizing membrane morphology have been defined and obtained by image analysis of electron micrographs. These parameters are geometric pore size distribution, effective pore size distribution, noncircularity of pore shape, surface porosity and fractal dimension of membrane pores. The fractal dimension of membrane pores was defined by the exponent of the relationship between pore perimeter and its equivalent diameter. The value of the fractal dimension (D) reflects the degree of irregularity of membrane pores. Different values of D show the different types of membrane pores and perhaps their different formation mechanisms. The results obtained are useful for the investigation of the relationship between membrane function and morphology, and for the study of the mechanisms of membrane pore formation.