Gold nanoparticles (AuNPs) with the sizes of 5.2, 6.7 and 7.3 nm have been synthesized by gamma-irradiation of Au3+ solutions with the corresponding concentrations of 0.25, 0.5 and 1.0mM using carboxylmethyl chitosan as stabiliser. The optical characteristics and particle sizes of AuNPs have been determined by UV- Vis spectra and TEM images respectively. The antioxidant activity of AuNPs has been investigated at the concentrations of 0.025mM and 0.5mM using ABTS scavenging activity. The results have shown that the higher concentration of AuNPs displays the stronger antioxidant activity and the faster reaction time. The highest antioxidant activity has been found at the concentration of 0.375mM within 2-3 minutes. The antioxidant activity of AuNPs increases by the increase of reaction time and is higher than that of ascosbic acid. Thus, gold/carboxymethyl chitosan nanoparticles prepared by gamma-irradiation method can be potentially utilized for the production of antioxidant products in pharmaceutics, functional foods and cosmetics.