Efforts to develop sustainable energy storage and conversion systems to replace fossil fuels have been conducted for decades. Oxygen-based electrochemical reactions at electrodes in these systems, however, have considerably slow kinetics, requiring appropriate electrocatalysts for their efficiency. Perovskite oxides are noteworthy materials substituting noble-metal-based catalysts. In particular, perovskite nickelates are promising electrocatalysts, which show high electric conductivity and remarkable electrocatalytic activities. For reported studies of oxide-based electrocatalysts, though the electrocatalytic reactions occur on the catalyst surfaces, they have demonstrated the relationship between electrocatalytic activities and structures, focusing on the bulk structures of the catalysts. In this thesis, we observe an atomic-scale structural change of both bulk and surface of perovskite nickelates when lattice defects are formed in them by using scanning transmission electron microscopy. Oxygens can be observed in annular bright field mode, enabling observation of transition-metal octahedron structures. Perovskite nickelates are epitaxially grown in thin films on single-crystal substrates to facilitate observing the lattice structure with a certain zone-axis. When lattice defects such as point defects and two-dimensional planar defects are formed, there are local structural changes in lattices, actually resulting in perturbation of oxygen atoms. We figure out the structural change of transition-metal octahedron via scanning transmission electron microscopy and the corresponding change in the electronic structure by DFT calculations and investigate the relationship between the lattice structure and the oxygen electrocatalytic activity.