Polystyrene/nanO-SiO2 composite microspheres (PS/nano-SiO2) with core-shell structure were successfully synthesized in a Pickering emulsion route using nano-SiO2 particles as stabilizers, which were organically modified by methacryloxypropyltrimethoxysilane (MPTMS) which containing a reactive C=C bond. The products were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), fourier transformation infrared spectrum (FTIR), thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). In addition, bare PS spheres could also be obtained by changing synthetic conditions. It was found that the morphology of the resulting products depends on particle concentration, particle wettability and pH value of particle dispersion. A possible mechanism for the formation of the composites with different morphologies is proposed. (C) 2008 Elsevier B.V. All rights reserved.