The effects of nano-rutile TiO2 and multiwalled carbon nanotubes on the growth and antioxidant enzymes of maize (Zea mays L. ) seedlings were investigated under hydroponic conditions. Nano-rutile TiO2 ( > or =50 mg/L) significantly decreased root and shoot growth (p < 0.05) after 9 days, while MWCNTs did not influence the biomass obviously. Under two stress conditions, the antioxidant system was activated respectively. Compared to the control, superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) were up-regulated (p < 0.05). Lipid peroxidation in terms of malondialdehyde (MDA) content was enhanced by TiO2 exposure, whereas, MWCNTs did not cause oxidative damages in maize seedlings. The results indicated that the phytotoxicity of nano-rutile TiO2 was much higher than that of MWCNTs, and oxidative stress induced by nanoparticles (NPs) exposure might play a role in the phytotoxicity of NPs. The remarkable effect of nano-rutile TiO2 is probably attributed to particle composition and shape.