Amphoteric latices were synthesized by emulsifier-free emulsion polymerization of mixtures of styrene, N,N''-dimethylaminoethyl methacrylate and methacrylic acid using potassium persulfate as the initiator. The size of latex particles as well as the size distribution were qreatly affected by pH of the system and concentration of the initiator. pH is an important factor for stability of the latex, which is stable either below pH 5 or above pH 8. The charge density on the latex surface ranges from 25 to 40μC/cm²a by means of conductivity measurement. Centrifugation of the neutralized latex system qreatly shortens the time required to find the isoelectric point.