The thermal stability of nickel monosilicide (NiSi) is one of the important research topics in the area of nano-complementary metal oxide semiconductor. This paper reports the effect of germanium (Ge) ion implantation on the thermal stability of the NiSi/Si structure. High dose Ge ion implantation (>5x10(15) cm(-2)) can improve the thermal stability of the NiSi/Si structure. Ge ion implantation before NiSi formation results in a very smooth NiSi/Si interface due to Ge atom pileup at the NiSi/Si interface. This high concentration Ge layer reduces the interface energy so that the thermal stability can be improved. Both the phase-transformation temperature and agglomeration temperature are improved by 50-100 degrees C. The effects of Ge ion implantation on the NiSi-contacted n(+)-p and p(+)-n shallow junctions are also examined. Although fast Ni diffusion via the ion implantation induced defects is observed, better thermal stability can still be observed on the n(+)-p junction.