We report a novel assembly approach for fabrication of molybdenum trioxide nanoplatelets with precise controls of stack architecture and crystallographic phase (either beta-MoO3 or alpha-MoO3, or their mixture) at 300-450 degreesC. The self-assembly of these metal oxides occurs within a predefined space of original hybrid single-crystals (i.e., the assemblies have a dimension as large as 50-250 mum), and an overall oxide assembly is strictly integrated throughout the original crystal space provided. In this sense, each single-crystal of the hybrid precursor can be viewed as a microscopic "green compact" analogous to the macroscopic one in ceramic processing.