In order to reveal the effects of single and combined treatment with different concentrations (0, 2, 4, 8, 16 mg·L^-1) of hydroxylated multi walled carbon nanotubes and 10 μmol·L^-1 Cd, we studied root growth physiological indexes and Cd content of Vicia faba seedlings by hydroponic method. The results showed that: (1) with the treatment of single MWCNTsOH, with the increase of MWCNTsOH concentration (2-8 mg·L^-1), the length of V. faba root increased, and the production rate of OO₂^-· increased along with the increase of SOD and POD activities. (2) The MDA content and O₂^-· production rate of seedlings treated with single Cd were both higher than that of the control group, and the O₂^-· production rate was 1.98 times higher than that of the control group. (3) The MWCNTsOH combined treatment induced the production of O₂^-· and the increase of MDA content in the root of V. faba seedlings. The 16 mg·L^-1 MWCNTsOH combined with Cd caused the greatest damage to the root of V. faba seedlings, and some root tip cells were damaged and shed, and Cd content was significantly decreased. It was found that low concentration of MWCNTsOH could promote root length and reduce the toxicity of Cd, while high concentration (16 mg·L^-1) of MWCNTsOH combined with Cd had a synergistic effect on root toxicity of V. faba seedlings.