Flow cytometry (FCM) is a new technique developed in the recent decade. This technique may measure DNA content of 5000 cells per second and trace the dynamic changes in cell proliferation cycle and offer a hint for designing clinical treatment protocol, monitor prognosis and elucidate the mechanisms of antitumor drugs. The authors previous studies showed significant effect of garlic oil on prolongation of life expectancy and inhibition of tumor growth in mice. Using FCM the authors analysed the effect of garlic oil on cell cycle in S180 tumor cells, 2-6 hrs after single administration or multiple administration the cell number in S phase rapidly decreased, in G1 phase increased. This suggests garlic oil may blockade cells to progress from G1 phase to S phase and result in accumulation of cells in G1 phase and directly inhibit the synthesis of DNA and the cell cycle. Theoretical basis for clinical application was offered and some aspects of antitumor mechanism of garlic oil were elucidated.