The present investigation aims to study the swift heavy ion irradiation induced effects in zinc ferrite nanoparticles through X-ray diffractometry, transmission electron microscopy and electron paramagnetic resonance spectroscopy. A series of zinc ferrite nanoparticles having different crystallite sizes ranging from 12 to 62 nm have been synthesized by using the nitrates of cations and the citric acid as the host. These samples were irradiated by 100 MeV oxygen beam with two fluences 1 x 10(13) and 5 x 10(13) ions/cm(2). The XRD patterns show the deterioration of crystalline phase after irradiation. The average crystallite size decreases after irradiation. EPR analyses of the pristine and irradiated samples show that the changes depend on the average crystallite size of the pristine samples. Further, the pristine sample having a crystallite size of 62 nm passes through dominant changes after irradiation. It was observed that the number of magnetic centres decreases in the irradiated samples. (C) 2010 Elsevier B.V. All rights reserved.