Monosized poly(styrene/poly(ethylene glycol) ethyl ether methacrylate/N-[3-(dimethyl-amino)propyl]methacrylamide) [poly(St/PEG-EEM/DMAPM)] monosize cationic nanoparticles were produced by emulsifier-free microemulsion polymerization conducted in the presence of a cationic initiator, 2,2'-azobis(2-methylpropionamidine)dihydrochloride (V-50), in which polymerization temperature, time and pH were changed. Particle sizes and surface charge densities were measured with a Zeta Sizer. Atomic force microscopy (AFM) was also used for measurement of particle size. The nanoparticles with the minimum size of 77.6 nm and with quite narrow size distributions (PI = 0.125), and with a zeta potential of about 54.4 mV (cationic) were produced by this method. The optimal temperature, time and pH were 60 degrees C, 3 hr and 2.5-3.0, respectively. It was possible to bind about 200 mg DNA per gram of nanoparticles at room temperature, in 2 hr in the pH range 3-8. Copyright (c) 2006 John Wiley & Sons, Ltd.