The properties of multiferroic BiFeO3 nanoparticles with core-shell structure produced by mechanically activated room temperature synthesis were studies by electron spin resonance spectroscopy and X-ray diffraction. The data reveal essential differences in ordered and disordered regions among as-prepared and annealed samples. The thermally-induced increase in the average size of the grains and disappearance of the disordered grain shell determined from the electron spin resonance and X-ray diffraction are in a good agreement and were confirmed by transmission electron microscopy observations. Ferro- and ferrimagnetic ordering was observed in as-prepared BiFeO3 nanograins, whereas antiferromagnetic order was apparent in the annealed material. The results show that the electron spin resonance spectroscopy can be considered as an useful method to determine the type of magnetic ordering in multiferroic nanoparticles.