Monodispersity is one of the important characteristics of magnetic polymer microspheres．In this paper

monodisperse magnetic poly(styrene／butyl acrylate／methacrylic acid) [P(St／BA／MAA)] microspheres with the size ranging from 80 to 230 nm

were prepared by emulsifier-free emulsion polymerization in the mixture of water and polar solvent

such as acetone

ethanol or methanol

in the presence of ferrofluid prepared by a co-precipitation method．The effects of the medium polarity and the surfactant content in ferrofluid on the particle size and monodispersity of magnetic P(St／BA／MAA) microspheres were discussed in detail．The results showed that the electrostatic repulsion between microspheres and radicals had a geat effect on the monodispersity of magnetic micropsheres．Proper repulsion benefited the monodispersity

but the higher or lower repulsion would lead to the decrease in monodispersity．Moreover

the results revealed that some features of emulsifier-free emulsion polymerization in the presence of ferrofluid was analogous to that of conventional emulsifier-free emulsion polymerization．