Nanoparticles of cobalt ferrite prepared by the co-precipitation method with crystallite size varying from 4.7 to 41 nm have been characterized by positron annihilation lifetime spectroscopy. Three lifetime components are fitted to the lifetime data. The shortest lifetime component is attributed to the delocalized positron lifetime shortened by defect trapping. The intermediate lifetime is assigned to the positron annihilation in diffuse vacancy clusters or microvoids at the grain boundaries and at the grain-boundary triple points. The longest component corresponds to the (...) pick-off annihilation of ortho-positronium formed at the larger voids. The variations in these lifetimes and their relative intensities with annealing temperature and crystallite size have been studied in detail. (shrink)