A newly synthesized nitroxide radical, 4-ferrocenecarboxyl-2,2,6,6-tetramethyl piperidine-1-oxyl (FC-TEMPO), was firstly encapsulated in the poly(DL-lactic acid) (PLA) nanoparticles by the modified spontaneous emulsification solvent diffusion method. The formation of the nanoparticles with diameters of 120 +/- 15 nm has been confirmed by a laser particle size analyzer. The morphology of the nanoparticles exhibited a fine spherical shape without aggregation or adhesion by the scanning electron microscopy (SEM). The electron paramagnetic resonance (EPR) spectra indicated that FC-TEMPO was mainly encapsulated into PLA nanoparticle core.