Scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM), equipped with an energy dispersive X-ray spectroscopy (EDS) were employed for examining the existing form of Cu in Zr-2.5Nb-0.5Cu alloys and the oxide films during corrosion testing in SCW (supercritical water) at 550 oC/25 MPa. In Zr-2.5Nb-0.5Cu alloy, Cu exists mainly in the form of tetragonal second phase CuZr2, rich in impurity element Fe. The CuZr2 are transferred and agglomerated with the β phase transformation of the alloy induced by hydrogen during the corrosion testing. In the form of amorphous cuprous oxide, Cu segregates on the grain boundaries of zirconia in the oxide films during corrosion testing.