A new kind of thermosensitive magnetic particles with core/shell structure was prepared by copolymerization of styrene (St) and N-isopropylacrylamide (NIPAM) in the presence of Fe3O4 magnetic fluid in ethanol/water medium. The particles shrank above the lower critical solution temperature (LCST) of Fe3O4/P(St-NIPAM) and swelled below the LCST in water. The effect of polymerization parameters such as initiator concentration and NIPAM/St ratio on particles was investigated. Higher initiator concentration formed larger Fe3O4/P(St-NIPAM) particles, and the particles size decreased as the NIPAM/St ratio increased.