Poly(glycolide-co-lactide) (PLGA) nanospheres containing magnetic materials have been extensively studied because of its biomedical applications. Therefore, it is very important to know thermal properties of these materials in addition to other physical properties. Thermal degradation activation energy (E-alpha) of PLGA nanospheres with maghemite entrapment (PLGA-Mag), PLGA nanospheres (hollow spheres) (PLGA-H) obtained by an emulsion method and unprocessed PLGA (PLGA-R) were calculated by isoconversional Vyazovkin method based on data of TG analysis in order to evaluate modifications in thermal behavior caused by nanospheres obtainment process or by maghemite entrapment. Both hydrodynamic diameter in the range of 200-250 nm and polydispersity index lower than 0.3 are considered satisfactory. Thermal degradation of PLGA-R begins at higher temperatures than those of PLGA-H and PLGA-Mag, but processed samples presented increase in thermal stability, which was greater before processing by emulsion and in the presence of the magnetic materials. PLGA-Mag presents superparamagnetic behavior at room temperature.