Abstract: bjective To explore the effect of up-regulated expression of microRNA-125a（miR-125a） on cell apoptosis of human lung cancer A549 cells and possible mechanisms. Methods The miR-125a sequence was synthesized and cloned into eukaryotic expression plasmid pGenesil-1 to construct recombinant plasmid pGenesil-miR-125a.Meanwhile，a control plasmid pGenesil control was constructed. The above plasmids were transfected into human lung cancer A549 cells using liposomes and divided into three groups： A549 cells transfected with pGenesilmiR-125（A549-miR-125a group），A549 cells transfected with pGenesil control（A549-control group）and A549 cells without transfection（A549）. Quantitative realtime polymerase chain reaction（QPCR） was used to measure the levels of miR-125a（normalized to U6 mRNA levels） after transfection. The percentage of apoptosis was determined by Annexin V-FITC／PI double staining with flow cytometry analysis. Western blot was employed to analyze the protein levels of p53. Results The recombinant plasmid pGenesil-miR-125a was verified by restriction analysis and DNA sequencing. The level of miR-125a in A549-miR-125a group was 2.72±0.41，higher than 0.97±0.16 of A549-control group and 0.96±0.11 of A549 group（P＜0.01）. The apoptotic rate of A549-miR-125a group was（31.04±2.48）％，higher than （6.91±0.72）％ of A549-control group and （6.73±0.56）％ of A549 group（P＜0.05）. The level of p53 protein in A549-miR-125a group was 3.91±0.46，higher than 1.01±0.06 of A549 control group and 0.99±0.04 of A549 group（P＜0.01）. Conclusion Up-regulated expression of miR-125a can promote the apoptosis of lung cancer A549 cells with the possible mechanism of the up regulation of p53.