Micron size Monodispersed polymethyl methacrylate (PMMA) and polystyrene (PSt) latices were synthesized by emulsifier free emulsion polymerization in single step. By suitable variation of the ionic strength, the monomer concentration, or the initiator concentration in the aqueous phase, the final particle sizes of PMMA and PSt ranged from 0 6μm to 1 8μm,and from 0 8μm to 2 2μm, respectively, with the distribution coefficient of particle size below 5%.(The mean diameter of latex particles increased with increasing the ionic strength and mononer concentration.On the contrary,it decreased as the initiator concentration increased for PMMA system.Therefore,a combination of low initiator,high ionic strength and hight monomer concentration is expected to result in larger size particles and narrower size distribution than ordinary emulsion polymerization.) The effects of the ionic strength and the monomer concentration in the emulsifier-free emulsion polymerization on the particle size are slight but significant for PMMA system versus PSt system under the same conditions.This is explained by the different nucleation mechanism of emulsifier-free emulsion polymerization of MMA and St.