The structure and the formation process of a Cu-Zn surface alloy formed on Cu(111) were studied by ultrahigh-vaccum scanning tunneling microscopy (UHV-STM) and low-energy electron diffraction-Auger electron spectroscopy (LEED-AES). An STM image of the Cu(111) surface after deposition of Zn at room temperature showed that the Zn atoms were localized in the vicinity of the step edges and have substituted Cu atoms. The density of Zn atoms decreased with increasing distance from the step edges. The migration of Zn atoms into the surface layer was tracked by STM measurements, at constant temperatures in the range 297-328 K. The migration rate was estimated to be, for example, 6.62 x 10(-3) s(-1) at 297 K. The migration rate of Zn atoms increased with increasing surface temperature, and the activation energy was estimated to be 24.7 +/- 9.0 kJ.mol(-1) from an Arrhenius plot. On the other hand, after annealing at or above 523 K, STM images showed that Zn atoms were distributed homogeneously into the Cu(111) surface layer, indicating that Zn atoms were randomly substituted into the Cu substrate. It was suggested that the alloying initially occurred around the steps and that the homogeneously spread Cu-Zn surface alloy was formed by the migration of Zn atoms. No ordered structure was observed for the annealed Zn/Cu(111) surface by STM and LEED.