In this study, spherical ordered mesoporous silica (s-OMS) was applied as a new type of nucleating agent in polystyrene (PS) foaming with supercritical CO2 as a blowing agent. These s-OMS particles were modified by the selective grafting of PS brushes on the outside surface, by which the mesoporous structure inside particles could be maintained. Transmission electron microscopy, X-ray diffraction, thermogravimetric analysis, Fourier transform infrared spectroscopy, and Brunauer-Emmett-Teller surface area analysis were used to characterize the structure of the original and modified particles; these indicated that the PS brushes were grafted on the outside surface and the inside porous structure were maintained. PS/s-OMS-PS composites were prepared by a solution blending method, and the s-OMS-PS particles could have been well dispersed in the PS matrix because of the surface modification. Subsequently, PS and composite microcellular foams were prepared by a batch foaming process, and the morphology characterization on these foams showed that the s-OMS particles exhibited an excellent heterogeneous effect on PS foaming. The heterogeneous effect became more significant when the foaming temperature or saturation pressure was low. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4308-4317, 2013