The association reaction between resveratrol (RSV) and free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH·) at different pH values has been firstly investigated by cyclic voltammetry. The voltammetric experiments showed that the main reductive signal of DPPH· gradually decreased with the increasing concentration of RSV. The stoichiometry of RSV-DPPH· complex was determined to be 1: 1 by means of amperometric titration method. Also, the association constants for this molecular complex at pHs of 4, 7.4 and 10 were calculated as 2.55 × 104, 4.96 × 104 and 7.28 × 104 M−1, respectively. At the same time, the antioxidant activities of RSV at different pHs were determined from its anodic peak potentials on cyclic voltammograms (CVs). According to the correlation between antioxidant activity and association complex constant of RSV, the association constants determined by voltammetric DPPH· assay can be used as a measurement of the antioxidant capability of RSV.