In present study, we carried out a systematic record of the relative antioxidant activity in selected Iranian medicinal plant species' extracts. The total phenol varied from 24.1 ± 1 to 289.5 ± 5 mg g -1 in the extracts. Flavonoid contents were between 25.15 ± 0.8 and 78.3 ± 4.5 mg g-1. 1,1-diphenyl-2-picryl hydrazyl (DPPH) radical scavenging effect of the extracts was determined spectrophotometrically. The highest radical scavenging effect was observed in Mellilotus officinalis with IC50 = 0.018 mg ml –1. The potency of radical scavenging effect of M. officinalis extract was about 4 times greater than synthetic antioxidant butylated hydroxy toluene (BHT). The greater amount of phenolic compounds leads to morepotent radical scavenging effect as shown by M. officinalis extrac