Single crystals ofNd1Ba2Cu3Oy(Nd123) high-temperature superconductors have been studied using an ultrahigh vacuum scanning tunneling microscope/spectroscopy system at room temperature. A square lattice with average lattice spacings of4×4 Å2has been observed on the surfaces of the single crystals without any surface protections or treatments. The as-prepared surfaces are found to be terminated by the CuO chain layer. Several surface defect structures presumably caused by the oxygen depletion are revealed. Among them, an one dimensional structural modulation with period of 1.5 nm alongbaxis is seen, which is suggested to be created by the ordering of the oxygen vacancies. The surfaces of Nd123 single crystals are proved to be highly clean and stable in air. Our results indicate that Nd123 single crystals are good materials for surface sensitive scientific researches and device applications. ©1998 American Institute of Physics.