The influence of the porosity of a filter medium and the metal fiber diameter of additional layers of filter medium with graded pore size on air permeability has been investigated. The relationships of the air permeability of the metal fiber filter media to the porosity and the pore size, and an empirical equation for the permeability, the porosity and the pore size are obtained using a statical method. The results show that the air permeability of the filter medium which consists of a control layer and additional layers of thick fiber with gradient pore size is much higher than that of the medium with uniform pore size under the same conditions. The air permeability coefficient of the filter medium increases with the increasl of the fiber diameter of additional layers. The obtained empirical equation can be used for evaluating the air permeability coefficient of the metal fiber filter medium with gradient pore size.Correspondent: Liao Jichang, Professor, Northwest institute for Nonferrous Metal Resesearch, Xi' an 710016, P. R. China, Tel: 08629 626749