There are an increasing number of studies on pharmaceutical properties of medicinal plants growing wild in various climates of Iran. However, there is little information available concerning antioxidant activity of medicinal plants growing wild in northeastern Iran. Thus, the aim of this study was to evaluate antioxidant capacity of the methanolic extracts from leaves of seven Iranian medicinal plants collected from the northeast. Evaluations were made for total phenols and flavonoids, 2, 2-diphenyl-1-picrylhydrazyl (DPPH) scavenging activity, reducing power, and lipid peroxidation inhibition. Overall, findings revealed that there were significant differences in antioxidant potencies amongst the tested plants. Among the plant samples, Berberis integerrima exhibited the highest DPPH inhibition activity (p < 0.05) followed by Mentha piperita and Berberis vulgaris, whereas Salvia officinalis and Foenciculum vulgare were found to be less effective DPPH scavengers (p < 0.05). Almost similar results were obtained in terms of reducing power, and total phenols and flavonoids. In lipid peroxidation assay, however, no plant sample reached 50% inhibitory concentration. Overall, B. integerrima exhibited a remarkable antioxidant potency in vitro. This result identifies this plant as a good candidate for further investigations.