The effect of transformations induced by annealing on the structure and magnetic properties of α-Fe₂O₃ antiferromagnetic nanoparticles synthesized by chemical route has been investigated by transmission electron microscopy, magnetization measurements and Mössbauer spectroscopy. Annealing of these samples re-crystallize the nanoparticles without changing its mean size modifying the crystalline anisotropy energy of the nanoparticles leading to a change in the reversible-irreversible regime and in the Morin transition behaviour. The nanoparticles re-crystallize without changing its mean size while annealing Recrystallization of nanoparticles modifies its crystalline anisotropy energy which produces changes in the reversible-irreversible regime and in the Morin transition behaviour.