Monodisperse poly[styrene-co-glycidyl methacrylate (GMA)] microparticles were synthesized by dispersion copolymerization in a water-ethanol medium. The effects of various polymerization parameters on the particle size and size distribution of the dispersion copolymerization were investigated. The dispersion of polymer particles decreased when the GMA was added if the polystyrene homopolymer particles were polydispersed. The GMA acted as a comonomer as well as a costabilizer in the dispersion copolymerization of styrene with GMA. The solvency of the monomer increased with the concentration of GMA in the polymerization medium because GMA has a greater hydrophilicity than styrene, resulting in a large particle size and a slow polymerization rate. From an HCl-dioxane analysis of the poly(styrene-co-GMA) microparticles, great amounts of epoxy groups were detected after the completion of dispersion copolymerization.