We report the simultaneous and selective synthesis of sing le-crystalline ConSi NWs (n = 1-3) and their corresponding crystal structures-simple cubic (CoSi), orthorhombic (Co2Si), and face-centered cubic (Co3Si)-following a composition change. ConSi NWs were synthesized by placing the sapphire substrates along a temperature gradient. The synthetic process is a successful demonstration of tuning the chemical composition in ConSi NWs. The synthesis and detailed crystal structure of single-crystalline Co2Si and Co3Si are reported for the first time including the bulk and the nanostructure phases. The electrical and magnetic properties of Co2Si NWs are investigated and compared with those of CoSi NWs.