Co-stretching processing technique was adopted in this paper for reducing ePTFE membrane pore size in hopes of bringing it the excellent retention property of very small particles. The ePTFE porous membrane were prepared from ePTFE powder through mechanical operations: extrusion, rolling, co-stretching of double-layer sheet in longitudinal direction, and transverse direction respectively and then thermosetting. The porous structures of ePTFE membrane were studied through SEM and Coulter Porometer. The results show that each layer of the double-layer ePTFE sheet can be joined together above 2.5 MPa stress at 210 degrees C, or 3.5MPa stress at above 190 degrees C in longitudinal stretching operation. The transition interface between the each layer in the base sheet (after longitudinal stretching operation) and in the membrane (after thermosetting operation) was formed, which contributes to the reduction in the pore size of ePTFE membrane.