1,1-Diphenyl-2-picrylhydrazyl (DPPH) radical-scavenging capacity and oxygen radical absorbance capacity (ORAC) of ten sweet potato cultivars with various flesh colors were measured. The DPPH radical-scavenging activity (DPPH-RSA) was almost the same among the sweet potato cultivars with white, yellow, and orange flesh ; however, purple-fleshed sweet potato cultivars showed about 10 times higher activity than those with other flesh colors. A similar tendency was observed for ORAC. Therefore, electron and hydrogen atom transfer reaction-based assays could both detect the high antioxidant activity of purple-fleshed sweet potatoes. Moreover, a high correlation between the DPPH-RSA and ORAC values was found for three cultivars and seventeen breeding lines of purple-fleshed sweet potato (R=0.894). Based on these results, our DPPH radical assay appears effective as an alternative method for estimating ORAC values of purple-fleshed sweet potatoes.