In this study, green synthesis of silver oxide nanoparticles (Ag2O NPs) was achieved by bio-reduction of AgNO3 using Thymus Capitatus leaves extract. The effect of silver precursor contraction (0.1–1 g, AgNO3) was studied. X-ray diffraction confirmed the crystalline nature of Ag/Ag2O NPs with a crystallite size range from 18 to 24 nm. SEM showed that the green synthesizing silver oxide nanoparticles having in general as cubical shape. The obtained Ag2O NPs exhibit absorption at 273 nm with a direct bandgap ranging from 3.87 to 4.02 eV and an indirect bandgap of 2.99–3.44 eV, according to Uv-visible spectra. FTIR analysis exhibit a two weak peak at 517 and 458 cm−1 attributed to silver oxide-NPs vibration, confirming the nanoparticles formation. Among the examined extracts, Ag2O NPs with 1 g weight of AgNO3 showed the highest antioxidant activity with value of 3.42, 18.98 and 18.97 mg/mL for DPPH, FRAP and TAC tests respectively.