A new method for the fabrication of spherical gallium nanoparticles (Ga-NPs) on diamond-like carbon (DLC) layers with high precision in their desired diameter and positioning is presented. The basic principle is the pre-patterning of a DLC film by focused Ga(+) ion beam irradiation and subsequent annealing. During thermal treatment the evolution of single Ga-NPs with spherical shape on irradiated areas is driven by phase separation and surface segregation of Ga from the supersaturated DLC layer. The shape and size of the implanted areas as well as the ion fluence serve as a Ga reservoir for the nanoparticle (NP) evolution which is strongly correlated with the NP diameter. For the formation of segregation seeds to avoid random segregation of the NPs small spots are additionally implanted with Ga within the irradiated areas. The NP evolution is then assessed with respect to the seed position and the material for the Ga-NP growth is gathered from the surrounding reservoir. Using this technique Ga-NPs were fabricated with a diameter ranging from 40 nm up to several hundred nm. Prospective applications, i.e. in the field of plasmonics, arise from the arrangement in chains as well as in periodical two-dimensional arrays with defined NP size and interparticle distance.