Abstract: Gold nanoparticles coated with 11-mercaptoundecanoic acid (mAuNPs) in diameter of 15 mm were synthesized using the ligand exchange method. The synthesized gold nanoparticles mAuNPs were characterized by transmission electron microscopy and Zetasizer. Then, the MTT assay was used to evaluate the toxicity of mAuNPs to mouse melanoma B16-f10 cells. All subsequent irradiation experiments were performed under a carbon ion beam with a linear energy transfer (LET) value of 50 keV/μm. First, coumarin-3-carboxylic acid (3-CCA) was used as a fluorescent probe to detect the radiation enhancement effect of mAuNPs on hydroxyl radicals in aqueous solution. Second, dichlorofluorescein diacetate (DCFH-DA) was used to evaluate the radiation enhancement effect of mAuNPs on intracellular reactive oxygen species (ROS). More importantly, the radiosensitizing effect of mAuNPs on B16-F10 cells under irradiation with carbon ions was assessed with the clonogenic survival assay. Our experimental results showed that mAuNPs had nearly no toxicity to mouse melanoma B16-F10 cells. The yield of hydroxide radicals in ultra-pure water in the presence of mAuNPs after exposure to carbon ions increased by a factor of 1.08~2.95. At a co-culture concentration of 5 μg/mL, mAuNPs increased the level of intracellular ROS and the sensitizer enhancement ratio (SER) of mAuNPs in B16-F10 cells at 10% survival level was 1.15. Thus, our study indicates that mAuNPs have radiation enhancement effect on melanoma cells under heavy ion irradiation.