The effects of 100 MeV Ni ion irradiation on magnetic properties of nanoparticles of Ni0.8Cu0.2Fe2O4 with average particle sizes of 40 angstrom and 60 angstrom, synthesized by chemical co -precipitation method have been studied. The spinel cubic structures were confirmed by XRD. The average particle size estimated by XRD and by Langevin function fitting are in good agreement for both the pristine and irradiated samples. The blocking temperature increases with particle size and does not change after irradiation. On irradiation by 100 MeV Ni ions, significant changes in the hysteresis loop features are observed, which may be attributed to formation of cluster of defects in the nanocrystalline samples due to swift heavy ion (SHI) irradiation. It is also found that SHI irradiation produces more dominant changes in the hysteresis loop of smaller particle size of 40 angstrom as compared to that of 60 angstrom. (C) 2007 Elsevier B.V. All rights reserved.