Objective:To investigate the effects of miR-21 inhibition on proliferation and apoptosis of A549/DDP cells. Methods:MiR-21 expression was examined in A549 and A549/DDP cell lines. The inhibitor of miR-21(ASO-21) was transfected into A549/DDP. MTT and flow cytometry were performed to analyze the proliferation and apoptosis after transfection,respectively. Computational software and miR predicting database were used to predict the downstream effectors of miR-21 in the lung cancer cell line. Results:①miR-21 expression was approximately 3 times higher in A549/DDP compared with A549 cells. ②Apoptosis was induced and proliferation was suppressed in A549/DDP cells when miR-21 was inhibited. ③Computational softwares showed that multiple genes are possible downstream targets of miR-21. Some proteins could possibly regulate miR-21. Those genes form a regulating network via miR-21,resulting in regulating cisplatin chemoresistance. Conclusion:MiR-21 inhibition leads to apoptosis and proliferation suppression of A549/DDP cells, and it may be a therapeutic target to reverse cisplatin resistance in lung cancer treatment.