Iron oxide nanoparticle (Fe3O4) has gained remarkable research interest for various applications, from environmental to biological, because of its superparamagnetic properties and good biocompatibility. In this work, the nucleation effect of Fe3O4 in a polylactide (PLA) matrix under an influence of an alternating magnetic field was studied. The nanocomposite films that is, containing different concentrations of Fe3O4 (similar to 8 nm) which were uniformly dispersed in the PLA matrix, were prepared via a solution casting method. The amplitude and frequency of the magnetic field impose great effects on the morphology and nucleation rate of PLA crystallization. Thermogravimetric analysis was used to study the thermal stability of Fe3O4/PLA and it showed that the thermal stability of Fe3O4/PLA was affected by the Fe3O4 content. Fourier transform infrared spectroscopy and X-ray diffractometry indicated that Fe3O4 shows impeding effect to the crystallization of PLA. Based on the differential scanning calorimetry results, composite with 1% of Fe3O4 content could promote the crystallization of PLA but it would become an obstacle when 3% of Fe3O4 was added. The result of polarized optical microscopy also showed a good agreement that the incorporation of Fe3O4 could act as an effective nucleation regent to the composite film. POLYM. ENG. SCI., 59:608-615, 2019. (c) 2018 Society of Plastics Engineers