The new potent nano-antioxidant of sulfur-containing butylated hydroxytoluene ligands (S-BHTLs)-conjugated with gold nanoparticles, Au-S-BHTLs, was synthesized by a conjugation of sulfur-containing ligands derived from BHT on the surface of gold nanoparticles (AuNPs). The in-house developing eight sulfur-containing BHT-ligands (S-BHTLs) were used for further study on functionalization with AuNPs and their biological activities. The antioxidant and cytotoxic abilities of eight types of Au-S-BHTLs such as compound Au-2a, Au-2b, Au-3a, Au-3b, Au-4a, Au-4b, Au-5a and Au-5b were tested by DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging and MTT assays against HT29 (human colorectal adenocarcinoma) and MCF7 (human breast adenocarcinoma). Based on the results, it was revealed that nanocomposites Au-5a and Au-3a attained over 50% lower IC values against free radical than the un-functionalized AuNPs. Meanwhile, nanocomposite Au-2b showed the highest cytotoxicity effect against both cancer cell lines. Selectivity and safety towards non-tumorgenic cell lines were also evaluated and proved superior selectivity index (SI) of all nanocomposites (SI > 2.00) against both cancer cells except Au-5b (SI = 1.87) against MCF7. Hence, the functionalization of S-BHTLs with gold nanoparticles has increased the selectivity of nanocomposites and enhanced the antioxidant potentials to become useful and promising anticancer agents. • 50