Sodium 2-olefinsulfonate (AOS) is used as reactive surfactant to prepare 10 nm around magnetic CoFe2O4 nanoparticles in advance. Magnetic P(St-BA-AOS)-coated CoFe2O4 composite nanoparticles has been synthesized in situ soapless emulsion. The effects of the amount of the initiator, monomer concentration and polymerizing temperature on the convention of monomer are investigated. So the polymerizing conditions are KPS=4.010-3mol/l St=0.75mol/l, mMP=20g/l, T=75 BA=0.15mol/l t=6.0h [AOS]=2% without any surfactant in the latex system. Results of IR and TEM analysis demonstrate that the desired polymer chains has been covalently bonded to the surface of CoFe2O4 nanoparticles, and confirmed that the composite particles have the core-shell structure. Copolymer P(St-BA-AOS) has a good hydrophile ability with functional groups such as -SO3Na and -COO- groups. The saturated magnetization of CoFe2O4/ P(St-BA-AOS) composite could reach 2.10emu/g, and show the characteristics of super-paramagnetism.