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In this study, the metallic alloy nanoparticles of silver and zinc oxide (Ag-ZnO) were biosynthesized by Capparis zeylanicaaqueous extract. The characterization of biosynthesized Ag-ZnO NPs was performed by scanning electron microscopy (SEM) and revealed that they were with irregular shape, whereas the Fourier transform infrared spectroscopy (FTIR) confirmed the role of secondary metabolites as capping and stabilizing agents on biosynthesized nanoparticles. The antioxidant activity of Ag-ZnO NPs was evaluated through 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging test, hydroxyl radical scavenging test, and reducing power assays. A DPPH free radical scavenging activity value of 340 (IC50µg/ml) was exhibited by Ag-ZnO NPs which is found to be higher than that of the ascorbic acid, i.e., 289.74 (IC50µg/ml). The hydroxyl radical scavenging activities of Ag-ZnO NPs and ascorbic acid were 344.35 (IC50µg/ml) and 316.10 (IC50µg/ml), respectively. In the case of reducing power assay, unlike DPPH and hydroxyl radical scavenging assays, Ag-ZnO NPs exhibited a slightly lower value (0.498) than that of the standard ascorbic acid (0.511). The reducing power assay displays a much less variation between Ag-ZnO NPs and standard (ascorbic acid), a dose-dependent effect was clearly observed in the reducing power for both sample and standard. The results obtained in the present study reveal that this biologically synthesized alloy nanoparticle, Ag-ZnO can be used as a potent antioxidant.