Polymer-nanoparticles composites with hollow structure were prepared by emulsion polymerization with Co,3,O,4, nanoparticles (OA-NP) added into emulsion system containing styrene (St), sodium dodecyl sulfate (SDS) and cetyl alcohol (CA) under irradiation using γ-rays. Co,3,O,4, nanoparticles were synthesized by hydrothermal method at first, and then modified with oleic acid (OA). The influence factors such as dosage of OA-NP and SDS on morphology and size distribution of hollow spheres were thoroughly studied. OA-NP was proved to inhibit the polymerization of St in the kinetic studies. It was found that pores occurred on hollow spheres’ shell at the first stage of reaction, and then the perfect shells formed gradually with polymerized time. The mechanism for the hollow spheres formation was proposed based on the obtained results. The soft magnetic properties of Co,3,O,4, nanoparticles were influenced directly by polystyrene.