This study aims to inversigate the effects of RNAi silencing of MDR1 gene and thus reverse the multidrug resistance of actue promyelocytic leukemia cells HT9 to allicin.One short hairpin RNA (small hairpin RNA, shRNA) was designed and constructed into pSilencer3.1-H1 neo plasmid.MDR1 shRNA expression plasmid pSilencer3.1-H1 neo MDR1 was constructed and introduced into HT9 cells.MDR1 mRNA was assayed by real-time PCR.The P-gp protein (encoded by the MDR1 gene) expressions were assayed by Western blotting.Cell mortality rate of HT9 cells was determined by the MTT method.After treated with allicin, the biochemistry feature of all cells was observed by gel electrophorosis,cell morphological alterations were observed through transmission electron microscope, and the cell cycles were detected by flow cytometry.MDR1 shRNA expression plasmid was constructed successfully.The MDR1 mRNA and protein expression were descreased significantly after pSilencer3.1-H1 neo-MDR1 transfected into HT9 cells.In HT9-shMDR1 cells, MDR1 mRNA were decreased [(0.110±0.005) vs (0.027±0.002), P<0.01], and P-gp protein were decreased [(1.454±0.027) vs (0.856±0.014), P<0.05];The sensitivity of transfected cells to allicin were increased significantly, IC50 were decreased from (52.75±0.64) μg/ml to (26.66±0.59) μg/ml.Compared with HT9 cells, DNA ladder existed in HT9-shMDR1 cells treated with allicin, the typical ultrastructure of apoptosis cell was found by electron microscope and apoptotic body could be seen.Compared with the control group, after treated with allicin for 24 h, HT9-shMDR1 cells in the S phase were significantly decreased [(31.40±2.13)% vs (53.80±1.87)%, P<0.01] and cells in G2/M phase were significantly increased [(35.62±2.06)% vs (9.37±2.09)%, P<0.01], However, cells in G0/G1 phase showed no significantly changes.shRNA expression plasmid pSilencer3.1-H1 neo-MDR1 inhibited the expression of MDR1 gene stably, and can significantly reverse multidrug resistance of actue promyelocytic leukemia cells HT9 to allicin.