Emulsifier-free emulsion polymerization of styrene (St) and copolymerization of St and 4-vinyl pyridine (4VP) in the presence of ammonium persulfate were studied. A comparison between the two polymerization systems was made. It was found that there were big differences comparing polymerization rate, the number and size of the particles and distribution, and molecular weight. For the St-4VP system, it was found that the additional amount of 4VP influenced the copolymerization of St and 4VP, molecular weight, and particle size. The formation mechanism of the particles was discussed.