A series of magnetic nanocomposites based on poly(ε-caprolactone) (PCL) and Fe3O4 nanoparticles were prepared using a facile in situ polymerization method.The chemical structures of the PCL/Fe3O4 nanocomposites were characterized by Fourier transform infrared (FTIR) spectroscopy.Wide-angle X-ray diffraction (WAXD) showed the incorporation of the Fe3O4 nanoparticles did not affect the crystallization structure of the PCL.Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the morphology and dispersion of the Fe3O4 nanoparticles within the as-synthesized nanocomposites.Differential scanning calorimeter (DSC) and polar optical microscopy (POM) showed that the crystallization temperature enhanced and the spherulites size decreased by the presence of Fe3O4 nanoparticles in the nanocomposites due to the heterogeneous nucleation effect.The thermal stability of the PCL was depressed by incorporation of Fe3O4 nanoparticles from thermogravimetric analysis (TGA).The superparamagnetic behavior of the PCL/Fe3O4 nanocomposites was testified by the superconducting quantum interference (SQUID) magnetometer analysis.The obtained biodegradable nanocomposites will have a great potential in magnetic resonance imaging contrast and targeted drug delivery.