Abstract: Objective To investigate the effect of small interference RNA(siRNA)targeting hoxa9 and low dose of cytosine arabinoside(Ara-C)on the proliferaion and apoptosis of human acute monocytic leukemia U937 cell line. Methods Effective and specific siRNA oligo targeting hoxa9 was designed and compounded.It was transfected into U937 cells by liposome.The expression of hoxa9 mRNA and protein were detected by reverse transcription PCR and Western blot.The effects of siRNA targeting hoxa9 and(or) low dose of Ara-C on the cell viability of U937 were detected by MTT assay.The apoptosis rate in each group was measured by Annexin V-FITC. Results The expression of hoxa9 was effectively silence by siRNA targeting hoxa9.As a result,the mRNA expression of hoxa9 and the protein decreased obviously.The inhibitory rate(IR)of cell proliferation in group “RNAi+low dose of Ara-C” was the most significant.And the apoptosis rate at 48 h in this group increased apparently,which was significantly different with those in the other groups(P<0.05). Conclusion siRNA targeting hoxa9 combined low dose of Ara-C can significantly inhibit the proliferation of U937 cells and promote its apoptosis.This work offers an experimental data for revealing the interaction mechanism and for the clinic applications of hoxa9 combined with chemotherapy in the treatment of leakemia.