Abstract: Carbon nanotubes filled with magnetic materials are very interesting as new materials for applications in biomedicine. A simple and efficient method was developed to encapsulate Fe₃O₄ nanoparticles in multiwalled carbon nanotubes (MWCNTs). Transmission and scanning electron microscopy, energy dispersive X-ray analysis, and X-ray powder diffraction measurements confirmed that the Fe₃O₄ nanoparticles are encapsulated in the MWCNTs. The magnetic properties of the MWCNTs and the Fe₃O₄-filled MWCNTs were measured using a vibrating sample magnetometer. Results showed that the Fe₃O₄-filled MWCNTs exhibited superparamagnetism at room temperature and possessed a higher saturation magnetization (Ms) (around 13.15emu/g) than that of the unfilled ones (around 0.35emu/g). The MWCNTs encapsulating Fe₃O₄ nanoparticles have potential applications in engineering and medicine.