We have investigated the microstructural and crystallographic evolution of nanocrystalline zirconia under heavy ion irradiation using in-situ transmission electron microscopy (TEM) and have studied the atomic configurations of defect clusters using aberration-corrected scanning transmission electron microscopy (STEM). Under heavy ion irradiation the monoclinic-ZrO 2 is observed to transform into cubic phase, stabilised by the strain induced by irradiation-induced defect clusters. We suggest that the monoclinic-to-cubic transformation is martensitic in nature with an orientation relationship identified to be (100) m∥(100) c and  m∥ c. By increasing the damage dose, both the formation of voids and irradiation-induced grain growth were observed. A model for the formation of voids is proposed, taking defect interactions into consideration. The study has also demonstrated that high resolution orientation mapping by transmission Kikuchi diffraction (TKD) combined with in-situ irradiation in a TEM is a powerful method to probe the mechanisms controlling irradiation-induced processes, including grain boundary migration, phase transformations and texture evolution.