Pure copper nanopowders are hydrogen-reduced in order to eliminate surface oxides and produce hierarchic architectures having inner-sponge structures with partially bonded nano/ultrafine particles and outer irregular-agglomerate boundaries. Due to a decrease in surface area by the particle bonding, the newly designed agglomerates exhibit improved surface stability after reduction, resulting in enhanced oxidation-resistance in the air at room temperature. For a comparative analysis, we also prepare two conventional micropowders having spherical and irregular particles. The compressibility of these three types of powders is analyzed using mechanical compaction. Finite element analyses on the compaction behaviors of the spherical and irregular particles are performed. The mechanical properties and microstructures of the compacts are investigated using microhardness tests, X-ray diffraction, and electron backscatter diffraction technique. Dislocation density, crystallite size, and grain size are correlated with the mechanical and compaction behaviors. From the analyses, three advantages of the hydrogen-reduced copper nanopowder are noted: (1) suppression of oxidation while maintaining nano/ultrafine structure of particles, (2) lower pressure required for high-density compaction than for spherical powders with nano scale, and (3) favorable fabrication of bulk nano/ultrafine structures without cracks or fracture.