To investigate the factors influencing the supercritical CO2 extraction of garlic oil, and to explore the inhibitory effect of allicin on human hepatoma HepG2 cell growth. Appropriate combination of process parameters for supercritical CO2 extraction of garlic oil was identified through orthogonal experiment. Human hepatoma HepG2 cells were cultured in vitro, and changes in growth of allicin-treated HepG2 cells were determined by MTT assay and flow cytometry. Optimal experimental conditions for supercritical CO2 extraction of garlic oil were extraction pressure of 15 Mpa, extraction duration of 1.5 h, and temperature of 40°C. Cell cycle analysis by MTT assay and flow cytometry showed that allicin had a marked inhibitory effect on growth of HepG2 cells, which increased with increasing allicin concentration and treatment time. Under inverted microscope, the allicin-treated HepG2 cells exhibited reduced intercellular junction and cellular shrinkage. With increasing concentration of allicin, changes in the intercellular junction and cellular morphology became increasingly evident. Most cells were detached; chromatins were condensed, compacted and split into clumps; and typical apoptotic characteristics such as apoptotic bodies emerged. Allicin has a certain inhibitory effect on growth of human hepatoma HepG2 cells.