Recently, interest in plant-derived food additives has grown mainly because synthetic antioxidants suffer from several drawbacks. Some plants are traditionally used for both food and medicine in Iran. Aqueous and methanolic extracts of Smyrnium cordifolium Boiss leaves and stems and Sinapis arvensis L leaves, stems and flowers were prepared and evaluated for their total phenolic content (TPC), total flavonoids (TF) and antioxidant activity. Antioxidant activities were evaluated by five different methods: free radical scavenging of 2,2-diphenyl1-picrylhydrazyl (DPPH), trolox equivalent antioxidant capacity (TEAC), ferric reducing antioxidant power (FRAP), inhibition of lipid peroxidation and reducing power. It was found that the leaves of S. cordifolium had the highest antioxidant activities (TPC: 51.31 ± 1.06 mg gallic acid equivalents/g dry weight, DPPH: 78.25 ± 0.69 % inhibition, FRAP: 381.17 ± 7.35 μmol Fe2+/g dry weight, TEAC: 96.27 ± 0.20 μmol trolox equivalents/g dry weight and TF: 19.00 ± 0.30 mg catechin equivalents/g dry weight) and thus could be rich sources of natural antioxidants. These results suggest that leaves of S. cordifolium may serve as a potential source of natural antioxidant for food and pharmaceutical application. Further research is needed to isolate, characterize, and identify the bioactive compounds present in these plants.