Abstract: The cytotoxicity of polyethylene glycol-modified carbon nanotubes (PEG-CNTs) on human embryonic kidney cells (293T) and hepatoma cells (HepG₂) was studied in vitro. The effect of the concentration of PEG-CNTs on the cell activity and survival rate was detected by a colorimetric assay method after exposure for 24, 48 and 72 h. The dead HepG₂ cells obtained by incubation in solutions with different concentrations of PEG-CNTs for 24 h can be stained by a dye, propidium iodide, and the cell mortality was determined by flow cytometry. Results show that the toxicity of PEG-CNTs with a good water solubility towards 293T and HepG₂ cells increases with increasing PEG-CNT concentration. When the concentration of the PEG-CNTs is less than 100 μg/mL, the toxicity of the PEG-CNTs is Grade Ⅰ(non-toxic) and when it is greater than 100 μg/mL but less than 200 μg/mL it is Grade Ⅱ(mildly toxic), according to the ISO2109932-5 cytotoxicity standard. The toxicity grade does not change with increased time.