Novel core/shell composite hollow spheres were successfully synthesized by a facile hydrothermal process. The composite spheres consisted of a hollow glass core and an iron oxide (alpha-Fe2O3) shell assembled by disk-like petals. By properly tuning the process conditions and the component of the reaction solution, a series of composite spheres with gradient in morphologies of the shell layer could be prepared. The products present wide open M-H loops (hysteresis loops) related to the special shell morphologies. With the building units of the alpha-Fe2O3 shells varied from spherical nanoparticles to disk-like nanopetals, the coercivity increased from 414.93Oe to 2201.90Oe. Based on a series of contrast experiments, the probable formation mechanism of the products was proposed. (c) 2009 Elsevier B.V. All rights reserved.