Fan’s paper on Cu nanowire synthesis using organic free radicals has been published on JACS. In this work, we report a new, general synthetic approach that uses heat driven benzoin radicals to grow ultrathin copper nanowires with tunable diameters. In-situ temperature dependent electron paramagnetic resonance (EPR) spectroscopic studies show that the active reducing agent is the free radicals produced by benzoins under elevated temperature. The nice thing is that the reducing power of benzoin can be readily tuned by symmetrically decorating functional groups on the two benzene rings. The controllable reactivity gives the carbon organic radical great potential as a versatile reducing agent that can be generalized in other metallic nanowire syntheses.