Ultrafiltration asymmetric porous membranes were imaged by two microscopy methods, which allow 3D reconstruction: focused ion beam and serial block face scanning electron microscopy. A new algorithm was proposed to evaluate porosity and average pore size in different layers orthogonal and parallel to the membrane surface. The 3D reconstruction enabled additionally the visualization of pore interconnectivity in different parts of the membrane. The method was demonstrated for a block copolymer porous membrane and can be extended to other membranes with application in ultrafiltration, supports for forward osmosis, etc., offering a complete view of the transport paths in the membrane.