Objective: To analyze the effect of cytarabine combined with vincristine on HL-60 cell line in vitro. Methods:The median-effect equation and MTT assay were used on HL-60. Results: The cytotoxic activity of cytarabine(Ara-C) and vincristine (VCR) used alone or in combination enhanced as drug concentration increased. The order of administration did not influence the cytotoxic activity of the combined antitumor drugs. The ratio of drug concentration was a factor to influence the killing effect. The interaction of the agents was synergistic at lower concentration, and antagonistic at higher concentration. Conclusion: The combined drugs interaction(CI ＜ 1)was synergistic at lower concentration and antagonistic at higher concentration. The ratio of drug concentration is a significant factor that can influence the killing effect.