The　nanosilica/polymethylmethacrylate　(SiO2/PMMA)　and　polymethylmethacrylate/nanosilica　(PMMA/SiO2)　core-shell　structure　composite　microspheres　were　fabricated　by　dispersion　polymerisation　under　similar　reaction　conditions.　These　two　kinds　of　microspheres　are　different　to　each　other　by　their　opposite　core-shell　structure.　The　composite　mircospheres　were　characterised　by　infrared　spectra,　transmission　electron　microscopy　and　thermal　gravimetric　analysis　(TGA).　The　diameter　of　SiO2/PMMA　particles　is　about　50　nm.　The　average　diameters　of　PMMA/SiO2　particles　are　in　the　range　of　60-70　nm　as　KH570　was　added　after　methyl　methacrylate　polymerisation　reaction　for　2　h.　The　TGA　curves　show　that　the　thermal　stability　of　SiO2/PMMA　is　better　than　PMMA/SiO2.