Using DPPH radical scavenging assay to investigate the antioxidant activity of crude methanol extracts from the leaves, flowers and pods of Cassia alata L. found that the leaf extract exhibited a stronger antioxidant activity than the extracts from the flowers and pods. On the basis of DPPH radical scavenging assay-guided isolation, the methanol extract of C. alata leaves was separated by silica gel vacuum chromatography and Sephadex LH-20 gel filtration chromatography afford a light yellowish powder (CA1), which was identified as kaempferol. This compound exhibited antioxidant activity (ED50 9.99 μM) that was six times stronger than that of BHT (ED50 57.41 μM) and fifty eight times stronger than that of emodin (ED50 578.87 μM).