Cationic copolymer acrylamide-dimethyldiallylammonium chloride[P(AM/DMDAAC)] wassynthesized from acrylamide (AM) and dimethyldiallylammonium chloride (DMDAAC) by dispersionpolymerization. The structures of polymers synthesized in different systems of dispersion were determinedwith infrared spectroscopy. The results showed that ethanol-H2 O was the best system of dispersion. Thecharacteristics of PDMDAAC-AM were studied by means of various instrumental analysis techniques，including transmission electron microscope(TEM)and Zetasizer 3000HSA. The results showed that theparticle size of cationic PDMDAAC-AM synthesized in the ethanol-H2 O system of dispersion was withinthe range of nanometer，and the dispersion polymerization in the emulsion state was helpful to dissipationof rea ction heat