Antioxidant activity of hydroxylated fullerenes, so-called fullerenols, against lipid peroxyl radical was evaluated byβ-carotene bleaching assay. All samples showed moderate to high antioxidant activity (%AOA), especially for C60(OH)12(70.1) and C60(OH)44(66.0) as compared with 8, 24, 26, and 36 hydroxylated ones (31.7–62.8). The detection of the possible products was conducted in the model reaction of both fullerenols and C60with methyl linoleate by MALDI-TOF-MS. These results suggested that the two possible mechanisms, such as C-addition to double bonds and H-abstraction from –OH groups, are involved in the present radical scavenging reaction.