The effect of modification induced by thermal treatment on the structure and magnetic properties of α-Fe₂O₃ romboedrical nanoparticles (≫ 30 nm) synthesized by chemical route has been analyzed by x-ray diffraction, transmission electron microscopy, magnetization measurements and Mössbauer spectroscopy. Annealing of these samples re-crystallizes the nanoparticles maintaining their size while changing the crystalline anisotropy energy of the nanoparticles thus leading to an increase of the spin reorientation Morin temperature and changes in the superparamagnetic-blocking behavior.