PMMA/SBA-15 composites were prepared from three different in-situ emulsion polymerization methods, that is, batch, semi-continuous and prior-immersion, which differed from incorporation of monomer methyl methacrylate and mesoporous SBA-15. The stability of the emulsion polymerization was investigated through monomer conversion, solid content, coagulum, particle size and particle size distribution. The structure, thermal, mechanical and dielectric properties of the composite were determined by powder x-ray diffraction, thermogravimetric analysis, differential scanning calorimetry, dynamic mechanics analysis and dielectric constant. The morphology of the composite was observed via scanning electron microscope. The results showed that the semi-continuous emulsion polymerization method possess the best stability of the composite emulsion. Compared with monomer prior-immersion in SBA-15, the composite made by the semi-continuous approach exhibited the highest storage modulus over 100oC, glass transition temperature and dielectric constant, whereas the prior-immersion method produced the composite with the lowest properties.