Antioxidant activity of extract-Agþ mixture and Ag NP solutions and supernatant was measured between 2. 5 and 3 mmol/g, while only extract was 6.66 mmol/g. Although recent study reported an increase in antioxidant activity when Ag was used [11], these results demonstrated that anthocyanin lost antioxidant activity when Agþ and Ag NP were present. We propose the potential scenarios to provide a persuasive mechanism for reduction of antioxidant activities that Agþ and Ag NP both may alter structure of anthocyanin and DPPH and somehow makes antioxidant property of anthocyanin ineffective or even lost and Ag NPs were synthesized using antioxidant power of red cabbage extract with its electrons donation ability to reduce Agþ ion to Ag0 for Ag formation NPs. Thus, we believed that Agþ ions and Ag NPs may promote super oxide radicals, which would tend to consume anti-oxidant capacity of red cabbage extract (anthocyanin). Effects of concentrations of AgNO3 and extract on the formation of Ag NP and antioxidant activity were investigated. Concentrations of extract acting as a reducing agent and Agþ as a monomer are critical for the formation of Ag NPs. Gibbs free energy must be exceeded for seed formation in nucleation process of Ag NP formation