Block copolymers spontaneously self-organize into nanometer-scale structures and are therefore of great interest for emerging nanomaterials and nanotechnologies. [1] Welldefined polymer aggregates, such as spheres, vesicles, rods, tubules, and lamellae, form in selective solvents. [1][2][3][4][5][6][7][8][9][10][11] Vesicles, which contain a cavity, are particularly interesting owing to their potential applications in encapsulation, drug release, and catalyst supports. [2, 3b,c, 4, 7] However, copolymer-derived nanoparticles with complex internal structure are less common. Eisenberg and co-workers discovered so-called large-compound vesicles (LCVs) in their family of crew-cut aggregates formed by self-assembly of asymmetric polystyrene-b-poly(acrylic acid) and polystyrene-b-poly(ethylene oxide) block copolymers in solution. [8] LCVs, which were supposed to be the fusion of many vesicles, consist of many small compartments separated by a copolymer matrix. Furthermore, meso-sized crystallike aggregates with internal hexagonally packed hollow hoops (HHHs) in the matrix have been reported. [9] These are the only examples of polymer aggregates with complex hollow structures as far as we know. [10] Moreover, if one segment of block copolymers bears reactive groups, the resulting aggregates can be fixed by chemical crosslinking. Such nanoparticles are shape-persistent, even with changes in the environment. Thus far, a few successful crosslinking reactions within the vesicle wall have been reported, such as photodimerization and polymerization. [3b,c, 4, 11] Herein, we report a simple approach to prepare novel organic-inorganic hybrid nanoparticles with a complex hollow structure. The method is based on the self-assembly of a reactive amphiphilic block copolymer, poly(ethylene