PSt/GMA/DVB microspheres were prepared by soap-free emulsion co-polymerization of St, GMA and DVB monomers. The fractional conversion of polymerizations was characterized by gravimetric methods. All the fractional conversions of polymerization exceeded 90 percentages. The amounts of epoxy groups on the microspheres were measured by hydrochloric acid- acetone method. The results showed that the added time of GMA monomers had little influence on the amounts of epoxy groups in the microspheres; however, with the increasing of GMA monomers, the epoxy groups increased; with the increasing of DVB monomers, the epoxy groups drastically decreased.