The transition-metal-free alkylation/cyclization of activated alkenes using Hantzsch ester derivatives as effective alkyl reagents is described. A wide variety of valuable oxindoles was constructed in a single step with excellent selectivity. The reaction occurs through the formation of alkyl radical species followed by the tandem addition/annulation of olefins under oxidative conditions. This protocol is expected to offer inspiration for developing novel and efficient applications of Hantzsch esters in organic synthesis.