Highly ordered nanoporous Co membranes were fabricated by a two-step replication from the honeycomb structure of porous anodic alumina. These metal membranes are confirmed to have two substructures: first, the Co membrane consists of very fine quantum-sized particles with diameters of about 2-5 nm; second, the very fine Co particles are assembled in a superstructure; i.e., fine and uniform channels 50 nm in diameter, more than 16 mm in thickness, having a pore density of about 10(10) cm(-2). New techniques were introduced into the two-step replication process. resulting in new features of the replicated metal membrane: high aspect ratio (more than 320:1), highly ordered pore arrays, and narrow size distributions of the pore diameters. These new techniques also lead to simplification of the fabrication process of metal membranes. Double-sided and single-sided Co membranes can be achieved simply by adjusting the electroless deposition time.