Core/shell microspheres of polymethylmethacrylate (PMMA) coated with Al2O3 were successfully fabricated by a mechanofusion system. The microsliperes were subjected to calcination to give hollow Al2O3 microspheres. The Al2O3 shell structure was destroyed during a pre-firing or sintering process without the addition of fine SiO2 powder. In contrast, addition of 5.0 or 10.0 Wt% SiO2 was found to be effective for maintaining the original shell structure, while the size of hollow Al2O3 microspheres increased slightly and a small hole formed in each microsphere during the removal process of PMMA.