The monodisperse submicron-size polystyrene(PSt) microspheres were prepared by emulsifier-free emulsion polymerization of styrene (St) using potassium persulfate (KPS) as initiator in ethanol-water mixed solvents.The effects of the concentration of KPS and St,polymerization temperature and the composition of the solvent on the size and morphology of PSt microspheres were investigated by scanning electron microscopy (SEM).The results indicate that the above factors evidently affect the diameter of the microspheres.The petal-like composite polymer particles were produced by emulsifier-free seeded emulsion polymerization in the presence of PSt seeds,styrene and acrylonitrile (AN) monomers using poly (ethylene glycol) (PEG) which has a methacryloyl group in the polymer end as reaction stabilizer.Such nonspherical shapes were controllable by change the ratio of St to AN.Such a surface morphology may provide a valuable information to elucidate the formation mechanism of the petal-like particles prepared by emulsifier-free seeded emulsion polymerization in the presence of PEG macromonomers.The result ulteriorly proved the formation mechanism of the unusual morphology for dispersion copolymerization system.