Co3O4 nanoboxes (see figure), prepared by a template-synthesis process and composed of compactly assembled cuboid Co3O4 nanocrystals, are reported. Heterogeneous nucleation and interparticle coarsening growth of the nanocrystals is proposed as the formation mechanism. These nanoboxes are characterized by their unexpected magnetic behavior, and may lead to the possibility of novel metal oxide hierarchical structures.