The role of xanthine in the growth process of Au nanoparticles (AuNPs) was clarified by Uv-vis absorption spectra, resonance light scattering (RLS) spectra, cyclic votlammetry (CV), and differential pulse voltammetry (DPV). In a growth solution containing 0.01 M phosphate buffer solution (PBS, pH 7.4), 24.3 μM HAuCl4, and 2 mM cetyltrimethylammonium chloride (CTAC), the AuNPs were produced by tannic acid (TA) reduction. The growth of AuNPs was inhibited by the addition of xanthine. The plasmon absorbance band of AuNPs is blue shifted, indicating that larger sized AuNPs were grown in the presence of xanthine. Both the absorbance at 555 nm and RLS intensity at 652 nm of the grown AuNPs decreased linearly with increasing concentrations of xanthine. The linear regression equation are A=1.865−0.016 Cxanthine and I=397.8−4.617Cxanthine for absorbance and RLS intensity, respectively. The results of CV and DPV reveal that the inhibition effect of xanthine on the growth of AuNPs may attribute to the formation of xanthine-Au(III) complex. The conditional stability constants of the complexes were determined to be 2.22×105.