Flexible high-loading nanoparticle-reinforced polyurethane magnetic nanocompositesfabricated by the surface-initiated polymerization (SIP) method are reported. Extensivefield emission scanning electron microscopic (SEM) and atomic force microscopic (AFM)observations revealed a uniform particle distribution within the polymer matrix. X-rayphotoelectron spectrometry (XPS) and differential thermal analysis (DTA) revealed astrong chemical bonding between the nanoparticles and the polymer matrix. Theelongation of the SIP nanocomposite under tensile test was about four times greater thanthat of the composite fabricated by a conventional direct mixing fabrication method. Thenanocomposite shows particle-loading-dependent magnetic properties, with an increase ofcoercive force after the magnetic nanoparticles were embedded into the polymer matrix,arising from the increased interparticle distance and the introduced polymer–particleinteractions.