The effects of Ni addition on the structure and various phase transitions of nanocrystalline Cu-Ge alloys were investigated for the first time. Cu 84Ge16-xNix (x = 0,1,3 and 5 wt.%) alloys were mechanically alloyed at room temperature and evaluated by means of X-ray diffraction, differential thermal analysis, thermal gravimetry, and scanning and transmission electron microscopy observations. Nanocrystalline α-Cu(Ge) solid solution was observed to form during the early stages of mechanical alloying. After prolonged milling times, it was partially transformed into the ζ-Cu5Ge intermetallic compound with an hcp structure. Increased quantities of Ni in the alloys not only delayed the formation of the intermetallic phase upon mechanical alloying and the nanocrystallization transition during annealing but also decreased the crystallite and particle size, which gave rise to nanocrystalline alloys with a high thermal stability at temperatures near their melting points. The mechanism leading to the high stability achieved was also investigated and described.