Hybrids and nanocomposites of polymer and magnetic Fe 3 O 4 nanoparticles have been utilized as magnetically-responsive materials and magnetically-directed nanoparticles. In this work, we prepare polymer-functionalized Fe 3 O 4 nanoparticles through in situ Diels–Alder polymerization using maleimide-functionalized Fe 3 O 4 nanoparticle as a precursor. Polybenzoxazine-functionalized Fe 3 O 4 nanoparticles (MNP-PBz) have been obtained and characterized with Fourier Transform Infrared, X ray photoelectron, and Raman spectroscopies. The high saturation magnetization value of 51.9emu g −1 of the MNP-PBz nanoparticles demonstrates its superparamagnetism. Moreover, MNP-FBz has been utilized as a nanofiller for preparation of cured PBz/MNP-PBz nanocomposites, which contain various MNP-PBz contents of 67, 50, 33, and 17wt.%. The sample of PBz/MNP-PBz-67 shows a storage modulus of 8.0GPa, a saturation magnetization value of 37.6emu g −1 , and a glass transition temperature above 380°C. As a result, the PBz/MNP-PBz nanocomposites could be classified as magnetically-responsive high performance materials.