AIM:To investigate the role of anti-miR-21 oligonucleotide ( AMO) in the anti-leukemic activity of decitabine (DCA) in vitro.METHODS:AMO and scramble oligonucleotide (SCR) were constructed and transfected into HL-60 cells.The miR-21 expression was analyzed by real-time PCR to identify the transfection efficiency .The cells were treated with DCA at gradient concentrations (0.5, 2.0 and 4.0 μmol/L) for 48 h.The mRNA expression of human period circadian protein 3 (hPer3) was detected by real-time PCR.The early apoptotic rates were determined by flow cy-tometry with Annexin V/PI staining.Mean fluorescence intensities ( MFI) of CD117 and CD11b were also measured by flow cytometry.RESULTS:The miR-21 relative expression level in AMO group was significantly lower than that in blank group and SCR group (P<0.01).IC50 of DCA in AMO group was significantly lower than that in blank group and SCR group (P<0.01).With the same concentration of DCA, the early apoptotic rate, the mRNA expression of hPer3 and the MFI of CD11b in AMO group were significantly higher than those in blank group and SCR group (P<0.01).The MFI of CD117 in AMO group were significantly lower than those in blank group and SCR group ( P<0.01 ) .CONCLUSION:Activation of hPer3 expression plays an important role in enhanced anti-leukemic activity of decitabine by AMO in vitro.