In order to identify the nature of small point defect clusters produced from cascade collisions, either made of interstitials or vacancies, an effective method is introduced in which the identification is made from the behavior of defects under electron irradiation. Material investigated is nickel neutron-irradiated with several facilities. Under electron irradiation, neutron-irradiation-induced defects having stacking fault tetrahedron (SFT) contrast shrank and disappeared, others having dislocation loop contrast grew. Majority of extremely small defects (≤ 1 nm), over 3/4 of all the defect clusters (bulk, 300 K) and about 95% of all the defect clusters (thin foil, 573 K), disappeared and are clarified to be of vacancy type. The size and spatial distributions of defect clusters in foil specimens and comparison of defect structures in thin foils with those in bulk disclosed the role of freely migrating interstitials.