The first example of a metal-free unactivated C(sp3)-H bond functionalization of alkyl nitriles with terminal vinylarenes to provide γ-ketonitrile derivatives is described. This protocol features simple operations, a broad substrate scope, and atom and step economy. In addition, Cu-catalyzed C(sp3)-H bond functionalization of azodiisobutyronitrile (AIBN) and analogues with terminal vinylarenes to generate γ-ketonitriles was also studied. A preliminary free-radical pathway was confirmed by capturing an alkyl radical, and a conjugate system was found that can stabilize radical intermediates and be in favor of this transformation. Density functional theory (DFT) calculations also provide important evidence of the free-radical pathway.