The copolymer of PLLA-PEG-PLLA was synthesized via ring-opening polymerization of L-lactide using polyethylene glycol as initiator, and the Fe₃O₄ nano-particles were prepared by thermal decomposition method. The PLLA-PEG-PLLA/Fe₃O₄ microspheres were prepared by organic solvent evaporation method, and the morphology and size of the microspheres prepared were observed by SEM. Vibrating sample magnetometer(VSM) and Tg were employed to investigate the magnetite content and magnetic properties. It is indicated that for the microspheres with the same particle size and different magnetic content, the higher the magnetite content is, the rapider the heating rate is. The heating rate of the magnetic microspheres with the magnetite content of 70.57% is the fastest, and the temperature could rise to a effective temperature of 42 ℃ for tumor hyperthermia. For the microspheres with the same magnetic content and different particle diameters, the smaller the particle diameter is, the rapider the heating rate is. The heating rate of the magnetic microspheres with the particle diameter of about 10 μm is the fastest.