Antioxidant activity of ten commonly consumed leafy vegetables in Thailand was determined using the β-carotene bleaching method. Neptunia oleracea exhibited the highest activity (13.1 mg BHA equivalent/ g dry weight) followed by Acacia pennata and Morinda citrifolia. Nine vegetables out of ten showed a higher antioxidant activity than 25 mg BHA equivalent/ 100 g fresh weight, while only one-third of the Japanese vegetables exhibited such a high activity. Extract from Neptunia oleracea gave five peaks (P-1, P-2, P-3,P-4 and P-5), which showed an antioxidant activity in HPLC analysis. It was suggested that P-3, P-4 and P-5 corresponded to the derivatives of apigenin based on the UV spectrum.