Helium ion beam induced cobalt nanowire deposition using dicobalt octacarbonyl (Co2(CO)8) as a precursor is described. 10 nm wide metal lines were fabricated with good repeatability and extremely high purity. The metal lines were deposited on electrical test structures to determine the nanowire resistivity and contact resistance. Measurements reveal that these metal lines have 50–100 μΩ-cm resistivity. The contact resistance of a Co line on a gold pad is 15 Ω with a 0.14 × 1.4 μm contact area. The resulting metal deposit size was determined by helium ion microscopy (HIM) because of its high imaging resolution, and further characterized by high resolution transmission electron microscopy (HR-TEM) and electron energy-loss spectroscopy (EELS). HR-TEM images reveal that these metal lines are composed of 6 ± 2 nm cobalt crystallite grains; and EELS analysis shows that no measurable carbon signal was observed. Single-line and multiple-line patterns were prepared to examine proximity effects around the metal line depositions. Both HIM imaging and electrical measurements of the patterns verify that the collateral proximity deposition between individual lines can be minimized. Finally, factors determining the ultimate line width achievable with the He ion beam are discussed.