Multi-walled carbon nanotubes (MWCNTs) were functionalized with a series of amino acids (lysine, arginine, cysteine, histidine, and aspartic acid) by sonication. Surface functional groups of the treated MWCNTs were investigated by infrared spectroscopy, Raman spectroscopy and thermogravimetric analysis. The results indicated the formation of various amino acid functionalities on the MWCNT surface, as well as the improved dispersion of MWCNTs in water. After functionalization, the antioxidant activity of all treated samples was analyzed using 2,2-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS), 1,1-diphenyl-2-picrylhydrazyl (DPPH), and hydroxyl radical scavenging, metal ion chelating, and reducing power assays. The antioxidant activity of the functionalized MWCNTs was 2–2.5 times greater than that of reduced glutathione (GSH) in ABTS radical scavenging, 1.5–5 times greater than that of GSH in reducing power, 1.3–1.8 times greater than that of butylated hydroxyanisole (BHA) in DPPH scavenging, and 3–10 times greater than that of GSH in hydroxyl radical scavenging. Accordingly, the amino acid-functionalized MWCNTs were appeared to be more potent than BHA and GSH synthetic antioxidants, and can thus be considered as excellent antioxidants to scavenge free radicals. © 2012 Wiley Periodicals, Inc. J Biomed Mater Res Part A, 2013.