We report a systematic theoretical study on the growth pattern and electronic properties of Zn12O12-assembled material using density-functional theory within a generalized gradient approximation. Our results show that assembly can form by attaching a Zn12O12 cage on a hexagonal site. A Zn12O12 cage should combine with eight hexagons in adjacent eight Zn12O12 cages, respectively, forming more stable assemblies. As the assembly process continues, we find that the Zn12O12 cages form a new three-dimensional nanoporous ZnO phase with a rhombohedral lattice framework. The Zn12O12 cage structure in the phase is preserved, and the Zn−O bond lengths between Zn12O12 monomers are slightly larger than those in the isolated Zn12O12 cage and the bulk wurtzite ZnO phase. The band analysis reveals that this new phase is a semiconductor with large gap value. Because of the nanoporous character of this new phase, it could be used for heterogeneous catalysis, molecular transport, and so on.