Abstract: The mechanism of nickel (Ni) metallization in silicon-germanium (Si0.5Ge0.5) alloy nanowire (NW) was studied. Transmission electron microscope imaging with in-situ annealing was conducted at temperatures of 200oC to 600°C. During rapid formation of Ni germanosilicide, loss of material from from the SiGe NW occurred which led to the formation of a thin Ni germanosilicide filament and eventual void. Energy dispersive X-ray spectroscopy analysis along the SiGe NW before and after annealing determined that Ge atoms tend to out-diffuse from the Ni germanosilicide towards the Ni source in the course of annealing. A model for the Ni germanosilicide formation in SiGe NW is proposed to explain this observation.