Magnetic crosslinked polystyrene (CPS) was prepared via a solvent-free surface-initiated atom transfer radical polymerization (ATRP) technique. Organically modified magnetite nanoparticles (MNP′s) were used as the polyaddition initiator. Also, an α,ω-bis-4-(vinylbenzyl)ether of polyoxyethylene (BVE) was utilized as the comonomer/crosslinker. In addition to Fe₃O₄-containing crosslinked polymer (donated with MNP′s@BVE-CPS), a non-magnetic polystyrene (denoted with BVE-CPS) was also prepared by the same technique, but without employing any nanoparticle. Magnetic properties of the initiator-anchored Fe₃O₄ nanoparticles (M-BnCl) and MNP′s@BVE-CPS thermosetting material were investigated using a vibrating-sample magnetometer (VSM). A reversible loop with sigmoidal shape and low coercivity (Hc) was observed, which showed the superparamagnetic behavior of these materials. The saturation magnetization (M_s) of MNP′s@BVE-CPS was found to be 34.0 emu.g⁻¹ at the external field of 10 kOe, while the 50% of the M_s occurs only in the field of 0.3 kOe. This showed that the sample is rapidly magnetized in relatively low fields. The crosslinked material possessing magnetite cores, i.e., MNP′s@BVE-CPS, clearly showed more thermal stability than the Fe₃O₄-unloaded material. In addition, the resulting thermosetting materials presented thermal stabilities nearly as much as other magnetite-containing polystyrene materials reported in the literature. Accordingly, the oxyethylene units arised from BVE crosslinker didn’t cause additional thermal sensitivity in the network obtained. Furthermore, differential thermal analysis (DTA) measurements indicated that the phase transition caused by the thermal decomposition in MNP′s@BVE-CPS composite is significantly reduced due to the presence of magnetite cores.