Objective To explore the influence of all-trans retinoic acid(ATRA) on in-vitro proliferation of human lung adeno-carcinoma cell line A549 and to preliminarily study its mechanism .Methods The human lung adenocarcinoma cell line A549 was taken as the experimental group .The cells in the experimental group and the control group were detected by using 3-(4 ,5-dimethyl-thiazol-2-yl)-2 ,5-diphenyltetrazolium bromide(MTT) assay for calculating the cell growth inhibition rate ;the microstructure of the cultured cells was observed by the transmission electron microscope technique ;the expression of cyclooxygenase-2(COX-2) in the cell supernate was detected by the enzyme linked immunosorbent assay .Results The different concentrations of ATRA for treating A549 cells could produce the inhibiting effect on A 549 cells to some different degrees ,furthermore ,after 72 h ,the expression of COX-2 in cytoplasma among the various concentrations groups was significantly decreased ,which was positively correlated with the concentration ,the difference showing statistical significance (P< 0 .05);at the same time ,by detecting TRAIL expression level in the experimental group(1 × 10-5 mol/L ,1 × 10-4 mol/) ,the expression of TRAIL in the A549 cell supernate after ATRA treatment was increased .Conclusion ATRA has the significant anti-tumor effect on the lung adenocarcinoma cell line A549 with the concen-tration-time dependence;ATRA in lung cancer tissue may play its anti-tumor effect by inducing apoptosis ,which can provide the theoretical basis for the treatment of lung cancer .