Abstract: To investigate the toxic effect of multiwall carbon nanotubes (MWCNTs) on the human hepatoma cell line HepG2 and to determine whether MWCNTs may affect the expression of metabolic enzymes GSTP1 and CYP1A1in HepG2, HepG2 cells were treated with 0.12, 0.25, 0.5, 1 and 2 mg/mL MWCNTs for 24, 48 and 72 h, and the effect of MWCNTs on the proliferation of HepG2 cells was then measured by thiazolyl blue colorimetry. Real-time cell analysis (RTCA), which accurately monitored the inhibitory effect on the proliferation of breast cancer MCF-7 cells with MWCNT treatment and also avoided interference of MWCNTs with the data, was used to monitor the proliferation of HepG2 cells treated with different doses of MWCNTs (0.25, 0.5, 1 mg/mL). After 48 h of treatment with MWCNTs, the mRNA and protein level of metabolic enzymes in liver were detected by a real-time polymerase chain reaction and western blotting. Results indicated that MWCNTs inhibited the proliferation of HepG2 cells in a dose and time dependent manner. The mRNA level and protein level of the metabolic enzyme GSTP1 in HepG2 increased while the mRNA level and protein level of CYP1A1 in HepG2 decreased in the cells treated by MWCNTs compared with the control group.