Nanoscience and technology are of great importance especially in terms of green synthesis of metallic nanoparticles, reducing the formation of by-products, chemical reagents and toxic solvents that have negative effects on the environment as well as on human health. In this study, iron oxide nanoparticles (FeNPs) were synthesized using Galium aparine L. extract. The nanoparticles were characterized by Ultraviolet-vis (UV-vis) spectroscopy and Fourier transform infrared spectroscopy (FT-IR). UV–Vis absorption spectrum of iron oxide nanoparticles display a peak in the region of 295–301 nm. FT-IR between 4000 and 400 cm−1 wavelengths exhibited exterior functional groups of FeNPs. The aim of this study was to evaluate antioxidant acitivities of FeNPs and Galium aparine L. extract. The antioxidant properties were evaluated using DPPH (1-1-diphenyl-2-picryl-hydrazyl), ABTS+ (2,2’-azino-bis (3-ethyl benzo-thiazoline-6-sulphonic acid)) and DMPD (N,N-dimethyl-p-phenylenediamine dihydrochloride) radical scavenging activity tests. According to the test results of DPPH (85.43%), ABTS+ (75.28%) and DMPD (68.25%), FeNPs prepared with Galium aparine L. has higher antioxidant activity than Galium aparine L.