Triblock copolymer micelles have been used successfully as templates to form porous; highly uniform, and hollow MoO3 nanospheres, with their diameter being equivalent to the micellar core size. By use of a combination of static and dynamic light scattering, small-angle X-ray scattering, and wide-angle X-ray diffraction, the MoO3 nanoparticles were found to crystallize into an extremely ordered simple cubic structure. The MoO3 was synthesized from the precursor compound MoO2(OH)(OOH) and formed a "thin layer" possibly at the interface between the micellar core and shell. The formation of MoO3 also affected the triblock copolymer micelles. The remaining bare nanospheres tended to aggregate into large aggregates. In the gel region formed by the triblock copolymers, the above process was so slow that the nanospheres were able to form before the core deformation. These nanospheres could subsequently be packed into a very ordered structure. The proposed mechanism can partly be suggested by the observation that the size of MoO3 nanospheres could be correlated with micellar core size. Therefore, the size of the hollow MoO3 nanospheres, over a limited size range, can be accurately controlled by choosing a suitable micellar core.